text:
Can I pay a "when this attacks, you may pay energy" cost multiple times?
So say I have an Aetherstream Leopard and I have 10 energy counters to spend, when I attack with it can I spend all 10 energy counters to give it a +20/+0?
And same thing with an Aether Swooper, spend 10 energy counters when attacking to create 5 servos?

What you have there are examples of triggered abilities. They say "Whenever [this card] attacks, ..." and this means that the moment you declare them as attackers, the ability triggers once and only once, and it's put on the stack.
When the ability resolves you follow the text of the ability: you either pay the mentioned energy cost or you don't. You don't have the option of paying it several times in a single trigger because the ability doesn't say you do. And it doesn't trigger multiple times because "Whenever [this card] attacks, ..." is specifically a simplification of "When you declare [this creature] as an attacker, ...".
You would have been able to pay an arbitrary amount of energy if either of the following had been the case:

If the ability were an activated ability. Activated abilities look like "Cost: Effect". So if the swooper had had "[2 energy symbols]: Create a 1/1 artifact servo token", you could've created an army at any time, provided you had enough energy.
If the triggered ability had explicitly let you pay as much energy as you wanted. If the leopard had had "Whenever Aetherborn Leopard attacks, pay any amount of [energy symbol]. Aetherborn Leopard gets +2/+0 for each [energy symbol] spent this way" you could've pumped it to be as large as you wanted, provided you had enough energy.

In the examples you asked about, no, these are triggered abilities that trigger once the creature is declared as an attacker during your combat step.  Either you pay the energy to get the effect, or you don't.  That being said, if you use a card to untap the creature and gain an additional combat during the turn (Aggravated Assault for example), this will allow you to declare the creature as an attacker again under the new combat within the same turn and trigger the ability again.  Assuming you pay the energy for the leopard's triggered ability each combat in a single turn, the leopard would grow in power each combat as the affect lasts till end of turn.
However, there are energy abilities that are activated abilities.  Aethersquall Ancient, Aethertide Whale, Aetherwind Basker, Architect of the Untamed and Bristling Hydra are just a few that have "Pay {cost}:" where the cost is an activated ability that must be paid by energy and can be activated any number of times in a single turn so long as you can pay the cost.  So the Aetherwind Basker could get +10/+10 until end of turn if you had 10 energy.

No. The instructions you must follow when the ability resolves only give you to option of paying {E} or nothing at all.
In the unlikely event that the ability triggers twice in the same turn (or if the triggered ability is somehow copied), you are given the option to make the payment again. If you do, it will get +2/+0, and that would stack with the bonus from earlier instances of the ability.

Think about it this way (above answers are correct anyway). When you attack with it, you fulfill its condition of "when it attacks". at which point the card asks you "would you like to pay [E]?" to which you answer "I would like to pay [EEEEEE]" the card would decline as the questions is "would you like to pay [E]?" asked once. It is a yes or no question, not an open question.
You pay [E], it resolves the ability of you paying [E] and you move on with your life.
A different example is Longtusk Cub where the wording is completely different. Look at both and you see why you can do so in one and not the other.

No, abilities written as "When/Whenever X you may Y" are triggered abilities, and they can only be used when that trigger is met, in this case, "Whenever Aetherstream Leopard attacks, you may pay . If you do, it gets +2/+0 until end of turn." triggers when Aetherstream Leopard is declared as attacking, resolves and can't be used again until the leopard is declared as attacking again. If you get a second combat step on your turn, generally through abilities on some red cards (like Aggravated Assault) you can activate it again when the Leopard attacks again, and since the ability is until end of turn the second attack will still have the benefit from the first attack's boost.
If they wanted you to be able to pay more into it, they could have written the trigger like this:

Whenever Aetherstream Leopard attacks, you may pay any amount of . If you do, it gets +2/+0 until end of turn for each  spent this way.

This is closest to the original ability, being still triggered, but lets you decide how much energy to spend all at once, when the trigger resolves, letting you pump all ten energy you have into the trigger.
They could have also changed it from a triggered ability into an activated ability, but kept restrictions on when it can be used, like this:

: Aetherstream Leopard gets +2/+0 until end of turn. Use this ability only before blockers are declared and only while Aetherstream Leopard is attacking.

This makes it an activated ability with restricted timing, when it's attacking, and only before your opponent declares blockers (so no post block tricks), which is slightly later timing wise but as close as possible to the trigger, but allows you to repeat the process as long as you can afford the energy.


text:
Removing a Commander permanently
So I was thinking of ways to get rid of Commanders other than destroying and exiling them. I found that some cards just plain out remove creatures and other permanents from game entirely. I was wondering if its possible to remove them that way because it's not like it went to the graveyard or exile.

As @Dopplegreener mentions, all cards that said "remove ... from the game" has been errataed to say "exile ...".
The problem with getting rid of a commander permanently is rule 903.9:

903.9. If a commander would be exiled from anywhere or put into its owner's hand, graveyard, or library from anywhere, its owner may put it into the command zone instead. This replacement effect may apply more than once to the same event. This is an exception to rule 614.5.

This rule lets the commander's owner put it back to the command zone no matter what happens.  Well, almost no matter what happens.

Ways to Nullify a Commander

The rule doesn't apply to phasing.  If you phase out a commander, such as with Vanishing, and have Stasis in play, their commander will be out of play until stasis is removed (phasing happens during the untap step, so Stasis prevents their commander from phasing in).  You can also have the commander out of play for most of the game without using Stasis by activating Vanishing as soon as their commander phases back in (their commander will exists for only a brief window at the beginning of some upkeeps).
If you take over their turn, such as with Mindslaver, and exile their commander on their turn, such as with Unmake, you chose whether or not to apply the replacement effect and can choose to have the commander be exiled rather than go to the command zone.
You can turn their commander into something else, such as with Song of the Dryads or Imprisoned in the Moon.  (Note that causing a commander to lose all abilities, such as with Lignify, isn't sufficient to nullify the commander against some decks, as a commander with no abilities is still a commander and can still deal commander damage.)
You can prevent them from casting their commander, such as with Meddling Mage
You can take control of their commander, such as with Empress Galina (your friends might not play with you if this is your commander) or Aethersnatch (it pains me that Commandeer doesn't work on most commanders; it can only target the few planeswakers that say "can be your commander").
You can eliminate the player that owns the commander, such as with Door to Nothingness.  When a player leaves the game, all objects owned by that player leave the game.

At the end of the day though, short of eliminating the player, you can't permanently prevent someone from accessing their commander1.  Any permanents you use to lock down their commander can be destroyed.  Any other tricks you pull can be negated through some combination of Pull from Eternity, Elixir of Immortality, Mastermind's Acquisition, and Brand.  There is always a way to get a card back, no matter where it has gone, unless it's owner has been eliminated from the game.
1. @ThomSmith found a way to permanently prevent a player from accessing a commander but it requires you to concede the game.

Joke Answers
There are a handful of cards not legal in the format2 that can either permanently remove a commander or that deserve honorable mentions:

The silver-bordered cards AWOL and Look at Me, I'm the DCI can put their commander into a zone not referenced by any other card in Magic.
Bronze Tablet (an ante card, not legal in any format) is one of the few cards that can change ownership of a permanent, nullifying rule 903.9.
During a subgame of magic, from Shahrazad or Enter the Dungeon, a commander is only available to a player if it was in the command zone at the start of the subgame ("719.2c As a subgame of a Commander game starts, each player moves their commander from the main-game command zone (if it's there) to the subgame command zone.") or if they pull a card from outside the game ("719.4. All objects in the main game and all cards outside the main game are considered outside the subgame (except those specifically brought into the subgame).")
If you make their commander a copy of the silver-bordered Chaos Confetti (such as by animating Chaos Confetti with March of the Machines and then using Cytoshape), take control of their turn (such as with Mindslaver) and activate the Chaos Confetti ability on their commander, you could destroy their commander card permanently (by ripping it to pieces).
Using the silver-bordered R&D's Secret Lair, you can remove cards from the game by playing an older version of a spell that exiles (such as Unmake) and ignoring the errata that changes this text to "exile".
If their commander is Mirko Vosk, Mind Drinker or Hokori, Dust Drinker, you could attempt to claim ownership of it via the silver-bordered Ashnod's Coupon (you'd probably need a judge to rule on this one).  If drink ever becomes a creature type, Ashnod's coupon will gain additional potency via cards such as Amoeboid Changeling and Unnatural Selection.
The silver-bordered Spike, Tournament Grinder lets you use any of these other banned cards (if only it weren't banned as well).

2. Some of these cards were briefly legal between Dec 1, 2017 and Jan 15, 2018.

While delightfully funny, Drinkers are not drinks.  Still getting an upvote.

Excellent list.  Surprised you didn't have "steal control" which is my personal favorite way to stop excessive commander shenanigans....

@AdamKlump I think it would be up to a judge.  They do literally have "Drink" in the name.  You would have to pay them the cost of the card of course (unless you had R&D's Secret Lair in play)

@John Good point.  I've added a point on stealing their commander, as well as an example of how that can be undone.

@Zags You mention research as an ability to pull cards from exile, but you can't actually do that anymore. "The exile zone is a part of the game, so you can’t get exiled cards." I only recently learned about this when trying to get Exiled cards using Masterminds Acquisition in MTGA

"If their commander is Mirko Vosk, Mind Drinker or Hokori, Dust Drinker, you could claim ownership of it via the silver-bordered Ashnod's Coupon" Besides the questions of whether those are drinks, Ashnod's Coupon just says the players "gets you" the drink. It doesn't say you get ownership of the drink.

@Malco Pull From Eternity is what gets cards back from exile.  Research was simply listed for completeness of cards that will get you a card back from anywhere (there's currently no effect besides a subgame that will move a commander to outside the game)

@Zags Now I’m curious how R&D Secret Layer interacts with Ashmod’s Coupon. Do you pay for the drink or not? It says play the cards as written; and he errata is written on the card. But it also says to ignore errata.

@GendoIkari It's a great question.  I've posted it along with my answer: https://boardgames.stackexchange.com/q/45073/9999

"such as with Lignify, isn't insufficient to nullify the commander against some decks" Is there an extra negative there?

@Acccumulation yep; edited

It sounds like you're describing cards that say "Remove (thing) from the game", like Unmake. Those are old cards from before the concept of Exile existed, and they now exile things instead. In the Magic 2010 rules change the Exile zone was introduced to the game, as was the action of exiling. With the same change, all cards that referenced "remove (thing) from the game" received errata to say "exile (thing)" instead. There are no current cards in the game that refer to removing anything from the game.
You can see that errata in cards that were reprinted later. On the left is the original pre-2010 printing of Unmake, on the right is the post-2010 printing with the instruction to "Exile target creature":

You can also see the current Oracle text of Unmake if you visit its Gatherer page, which is always the official source of what a card is truly considered to say, including all errata. You'll see that errata is reflected even on visiting its original Eventide printing:

Commanders can't get "removed" permanently very easily at all: that's part of the point of the format. Typically if removing them to the Command zone isn't enough, you instead lock them down via auras such as Darksteel Mutation, Song of the Dryads, or One Thousand Lashes, or you steal control of them—but that only lasts until the player finds appropriate removal.

The most permanent way to get rid of a commander (not counting AWOL) is probably to shuffle the commander into their library while you're controlling your opponent's turn.  Since you control your opponent's actions you can choose to skip the option to move the commander back to the command zone, and once it's in the library they can't get it out without tutoring or drawing it.

@ArcanistLupus If the game allows you to control that effect then yes, that would work, but in that case just allow unmake or some other permanent exile to resolve on the commander, don't even let it go back into the deck.

@Andrew Oh, good point.

I disagree that even AWOL would work. AWOL's first sentence is "Remove target attacking creature from the game" This would be subject to the exile replacement effect, then the further instructions to further remove it would fail to find it in exile. Errata on AWOL changes that first ability to "Exile target attacking creature."

@Andrew Oh hey you're right, I somehow thought they specifically left that reference in without errata for exile, but they must have added that errata with the Unstable silver bordered oracle updates. :P

Yeah the silver borders received errata some time last year, I think around the time they were made EDH legal (for a few months)

@Andrew Yeah, with the release of Unstable. I thought they deliberately left that part of AWOL un-errataed, but evidently I was mistaken.

Yes.
Can this sequence of events cause a commander to be permanently phased out?
Specifically, you can cause another player's commander to phase out and never phase back in as long as that player is in the game. This means that the commander is totally inaccessible; there is no sequence of actions that could put it back into play.
Suppose that you have a three-player Commander game. The players are Alice, Bob, and Carol. Bob's commander is Isamaru, Hound of Konda, a vanilla creature.
On Alice's turn, she:

Casts Dominate to gain control of Isamaru.
Casts Song of the Dryads to turn Isamaru into a colorless Forest land (that's not a creature).
Activates the first ability of Memnarch to permanently turn Isamaru into an artifact (an Artifact Land — Forest).
Casts Bludgeon Brawl, turning Isamaru into an Artifact Land — Equipment Forest.
Casts Grizzly Bears (a vanilla creature) and equips Isamaru to Grizzly Bears.
Casts Donate to give Alice control of Isamaru.
Casts Reality Ripple to phase out Grizzly Bears.
Concedes the game.

When Grizzly Bears phases out, Isamaru phases out indirectly (702.25f). Because it phased out indirectly, it will not phase out by itself. Instead, it would phase in at the same time as Grizzly Bears. When Alice concedes, Grizzly Bears ceases to exist without ever phasing in.
If Alice had not Donated Isamaru to Bob, then rule 702.25m might have caused Isamaru to phase in during the next player's untap step. However, Bob controlled Isamaru as it phased out, so 702.25m does not apply.
Obviously, this is highly impractical — but it does work.
Before the C17 rules change, there was a slightly easier approach. Instead of equipping Isamaru to Grizzly Bears, Alice could attach it to a token. Under the older rules, tokens ceased to exist when phased out, so Alice would not have to concede. Under the current comprehensive rules, tokens do not cease to exist when they phase out, so this approach would not work. As far as I know, the only way to prevent a permanent that phased out directly from ever phasing in is for the permanent's owner to concede the game, which limits this approach to a multiplayer game.

This is not strictly true if you're playing without the banlist. A sub-game (as created by Saharazad) causes all of the cards in the main game to be considered "outside of the game". You can then utilize Mastermind's Acquisition to bring the commander into the subgame. Upon the end of the subgame, the commander will be shuffled back into your deck, thereby escaping it from being phasing out.

You can't get cards from outside the game in Commander (rule 10).

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

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text:
What cards can I cast from exile with Dream Pillager?
Dream Pillager, after damaging an opponent, allows you to exile that many cards from the top of your library and cast any until end of turn that were exiled this way. Does this mean I may cast ANY card that has EVER been exiled this way since the beginning of the game?

Related, possibly a duplicate: https://boardgames.stackexchange.com/questions/41878/is-this-way-defined-in-the-rules?r=SearchResults

No, you can't cast any card at all that's in exile that way. You can only cast cards that were exiled by Dream Pillager this turn, and only until the end of the turn.
Dream Pillager's original printed text says exiled this way, referring to the cards exiled by that ability. Its wording was updated with Core 2019 to the following, emphasis added:

Whenever Dream Pillager deals combat damage to a player, exile that many cards from the top of your library. Until end of turn, you may cast nonland cards from among those exiled cards.

In making this change they were trying to make it extra clear you only get to cast that bunch of cards just exiled, only this turn.

For comparison, see Theater of Horrors for the syntax of a card that lets you play any card it has exiled if you've met the condition this turn.

Compared it to the -X ability on Ashiok, this lets you cast anything castable that was exiled this way this turn until the end of turn, but you have to pay the right amount of mana and correct colours (there are cards that will allow you to pay any mana as the mana for it like Ghonti)

It says "Until end of turn, you may cast nonland cards from among those exiled cards."
This is implying that you can only cast the nonland cards that you exiled during that turn.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Can I use two different cards with the same name in Commander?
I have two cards with the same name but different art and the card descriptions on the bottom. The card in question is Forsaken Sanctuary. It's a white/black land and I'm not sure if I can use both of them, it is the first time I've ran in to this.

No, card art or any other piece of information printed on a card doesn't matter. For card identity, the only factor is the English name being equal or not.
There are several reasons why cards with the same name can look different in print. They can be translated into another language. They can have alternate art in the same set (e.g. Hymn to Tourach). They can be reprinted in a different set, potentially with new art and/or general card style. Cards from different sets can have different printed characteristics, such as differently worded abilities, or different types (see THE GRAND CREATURE TYPE UPDATE
) due to changes in the rules or general wording conventions. Very early cards can even have misprints, such as Blue Hurricane.
However, ultimately the only factor in deciding whether or not two cards are identical is their English name; on a related note, the official characteristics of a card can be found on the Oracle website

108.1. Use the Oracle card reference when determining a card’s wording. A card’s Oracle text can be found using the Gatherer card database at Gatherer.Wizards.com.
201.2. A card’s name is always considered to be the English version of its name, regardless of printed language.
903.5b Other than basic lands, each card in a Commander deck must have a different English name.

Wow, 201.2a looks redundant. If you go so far as to define what "the same name" means, why aren't they also defining what it means for two names to be identical?

@Arthur a) because "same name" is a key term used in card wordings so it has to be defined, whereas "their names are identical" is the common, English meaning that needs no further definition; and b) because the full definition of "same name" is a little more complicated than that. "201.2b If an object has more than one name, it has the same name as another object if there are one or more names that both objects have in common."

@Arthur It isn't redundant. Consider two different versions of the same character Akroma, Angel of Wrath and Akroma, Angel of Fury or most planeswalkers, etc. The monster's name could just be considered "Akroma".  But that wouldn't stop you from having an "Akroma, Angel of Fury" and an "Akroma, Angel of Wrath" out at the same time. English is a goofy language where common practices make things more complicated in a game like Magic.  So they specified so that there wouldn't be any pedantic English language arguments during games.  Most rules are spelled out to the n-th degree.

You might like to cite commander rule 5: "With the exception of basic lands, no two cards in the deck may have the same english name. Some cards (e.g. Relentless Rats) may have rules text that overrides this restriction." That even handles functional variants from Unstable.

Does this include silver-border cards like Ineffable Blessing that have multiple versions with different texts?

@user3490 Here's the EDH rules committee response: legality is by card name, so only one of a functional variant (you can't have two Ineffable Blessings even if they're different) but one each of the Killbots because they've got different names.

The limitation on deck construction is by card name, not art or edition.  So you can't run two cards named Forsaken Sanctuary.  You can, however, run other cards that do the same thing but have different names.  Forsaken Sanctuary has a lot of equivalent cards under different names, most of which are strictly better, such as:

Concealed Courtyard
Isolated Chapel
Orzhov Guildgate
Scoured Barrens
Shambling Vent
Temple of Silence
Godless Shrine
Scrubland

108.1. Use the Oracle card reference when determining a card’s wording. A card’s Oracle text can be found using the Gatherer card
  database at Gatherer.Wizards.com.
201.2. A card’s name is always considered to be the English version of its name, regardless of printed language.
201.2a Two objects have the same name if their names are identical.

Art doesn't matter - you can have two completely different cards altered (painted) to look exactly the same, yet as the name is different, they are different cards. (altering is quite common in commander decks)
The text below, possibly rules text or flavor text can be different on different editions. As we travel through Ravnica in the current set, we can have reprints of cards that were printed back in Theros block. As Theros was heavily God-related plane, you would have mentions to Gods in the flavor text, as opposed to Ravnica which is more about societal guilds. That is one of the reasons for the differences, yet they are the same card.

Could you update your answer to stand alone and cite the relevant rules itself? This is a decent explanation, but it being a tack-on answer means I'm not inclined to upvote it. (Answers are at their best when they can be taken as an answer independently without needing to read others.)

@doppelgreener this is done

You can't use both but that you can do it pick up the other dual lands like shock lands or even buddy lands for the colors.


text:
Void Winnower against Sunforger
Does the use of Sunforger's unattach ability negate the ability of Void Winnower and let me use spells with an even converted mana cost?

No you can not cast CMC 0, 2 or 4 spells with Sunforger while your opponent controls a Void Winnower.
Sunforger's ability gives your CMC 4 or less spells an alternate cost, but it doesn't actually change their CMC.

117.9: Some spells have alternative costs. An alternative cost is a cost listed in a spell's text, or applied to it from another effect, that its controller may pay rather than paying the spell's mana cost. Alternative costs are usually phrased, "You may [action] rather than pay [this object's] mana cost," or "You may cast [this object] without paying its mana cost." Note that some alternative costs are listed in keywords; see rule 702.
117.9c: An alternative cost doesn't change a spell's mana cost, only what its controller has to pay to cast it. Spells and abilities that ask for that spell's mana cost still see the original value.

So even though you aren't actually paying for the spell, Void Winnower's ability still sees it as an even Mana Cost and prevents you from casting it.
Interesting side note brought up by @ikegami in the comments: Even though you can't actually cast the spells, you can still attempt to use Sunforger's ability, so if you will be able to pay RW, unattach Sunforger, look through your library, then shuffle it. Might be useful if you need to shuffle your library for some reason (ie. you control an Experimental Frenzy and have a land stuck on top).

Void Winnower prevents you from casting cards with even mana cost via Sunforger.

Why Sunforger can't cast cards with even converted mana cost
The reason is from the Magic Golden Rules:

101.2. When a rule or effect allows or directs something to happen, and another effect states that it can't happen, the "can't" effect takes precedence.

Sunforger tells you to cast it.  Void winnower says you can't cast it.  Rule 101.2 means Void Winnower's ability takes precedence.

Why Sunforger can still cast cards with odd converted mana cost
So can you cast anything with Sunforger while Void Winnower is on the battlefield?  At first glance, it seems like Void Winnower would prevent you from using Sunforger at all because it prevents casting cards with converted mana cost 0 and Sunforger makes the spells cost 0.  However, this is not the case.
Sunforger creates an alternative mana cost, which doesn't change the converted mana cost of the card, so you can still use Sunforger cast cards with converted mana cost 1 or 3 while Void Winnower is on the battlefield.
The relevant rules are:

117.9c: An alternative cost doesn't change a spell's mana cost, only what its controller has to pay to cast it. Spells and abilities that ask for that spell's mana cost still see the original value.
202.3. The converted mana cost of an object is a number equal to the total amount of mana in its mana cost, regardless of color.

101.2 not really what is applicable on this case. There is nothing allowing the casting of a 4 CMC in the same sense that winnower prevents things to be cast. Sunforger is creating an alternative casting method

@fireshark519 Sunforger says you can casting it (just without paying it's mana cost).  Void winnower says you can't cast it.  This is exactly what's relevant.

@fireshark519 101.2 absolutely applies in this case, your own answer applies 101.2 but doesn't quote it. "you would be casting the spell, whether you pay the mana cost or not, it will still be a spell with converted mana cost 4 (or 2)."

@Zags - rectracted my -1. Even though I don't agree as Sunforger is not allowing you to cast a spell but instead the ability is to cast a spell, I don't agree that it creates this point in the rules where there is a "can" and "can't" In my opinion it is more applicable to a Bristling Boar with menace than this.

@Zags is correct. The rules direct you to follow Sunforger's ability's instruction to cast the spell (608.2c). Void Winnower's effect states you can't. Winnower's effect takes precedence over the rules (101.2). You do as much as possible (609.3), so you end up searching your library and shuffling it. (The card you found during your search is shuffled too since it never left the library.)

Re "Why Sunforger can still cast cards with odd converted mana cost", Nice addition

No it will not. Sunforger's unattach ability reads:

Unattach Sunforger: Search your library for a red or white instant
  card with converted mana cost 4 or less and cast that card without
  paying its mana cost. Then shuffle your library.

As you can see by the wording on the ability you would be casting the spell, whether you pay the mana cost or not, it will still be a spell with converted mana cost 4 (or 2). You can still use it but only for spells with cmc 1 and 3.


text:
Can Chandra, Acolyte of Flame cast Heartfire without the additional cost?
I was under the impression that if you used Chandra, Acolyte of Flame's -2, targeting Heartfire, you would still have to sacrifice a creature, but someone told me that was not true. What is the ruling here?

Chandra's ability does nothing to change the total cost of Heartfire, which includes sacrificing a creature.

You may cast target instant or sorcery card with converted mana cost 3 or less from your graveyard. If that card would be put into your graveyard this turn, exile it instead.

Note that it doesn't modify the costs, only where you can cast the card from (in this case, allowing you to cast it from your graveyard).  Compare the phrasing to Aluren, which allows you to cast specific creature cards for free.

Any player may cast creature spells with converted mana cost 3 or less without paying their mana costs and as though they had flash.

Note that even if Chandra had the phrase "without paying it's mana cost" you would still have to sacrifice the creature as the sacrifice is an additional cost, not a mana cost.

You still need to sacrifice a creature
Chandra, Acolyte of Flame's -2 ability gives you the ability to cast an instant or sorcery from your graveyard, but you still need to pay costs for it. This includes the mana cost, as well as any additional costs. Sacrificing a creature is an additional cost.
The ruling at the bottom of the card's Gatherer page (linked above) mentions this specifically.

You must pay the costs for the spell you wish to cast with Chandra’s last ability. If that spell has any alternative costs, you may pay those.

Here 'may' means that you have the option of paying optional costs (such as kickers). However, if the additional costs are not optional, you must pay them.


text:
What is the statistically favorable amount of lands for Yennett?
Recently I have been brewing up Yennett, Cryptic Sovereign, as a commander deck. The deck was going well, with only 3 cards with even mana costs, and those being  Lightning Greaves, for protection, and Strionic Resonator and Helm of the Host, which serve to increase my statistical chance of hitting an odd card with Yennett's trigger.
This was fine until I remembered that lands have an even converted mana cost, since 0 is considered even in magic. So now I run into a problem where to increase my chances past 60% or so, I need to decrease my land count. This can be compensated for by replacing lands with odd CMC mana rocks, but this still makes my deck low on lands and still dependent on them, as most of the mana rocks would end up at 3 CMC. To allow my deck to function, it needs Yennett on the field.
This leads to a few questions:

What is my optimal amount of lands to increase my chances to a consistent amount and still have the mana to make my deck function?  How few lands can I realistically go?
What should I do to compensate for the lack of mana?  Which mana rocks are worth it?
Can this be compensated for with library manipulation and draw such as Ponder, Brainstorm, and Portent?  If so, how many of these small cantrips must I run to make them effective?

This question has several issues. Most importantly it is impossibly broad, with multiple unrelated questions.

I'm curious as to why you want to push your chances of hitting an odd CMC card so high (I understand that you want to free cast as much as possible).  If you were to draw a land with Yennett, you could just play it immediately.  Also, there may be times where you can't / don't want to attack with Yennett, so you may have to hard cast things.  I guess what I'm wondering is, why are you willing to destroy / weaken your mana base to be able to maybe free cast a couple of rocks?

Agree with @Becuzz, drawing a 0 CMC card is virtually identical to being able to cast it without paying its mana cost anyway - in most cases, it's better, since you don't have to cast it this turn. Yennett doesn't help you get around 1 land per turn.

"0 is considered even in magic" 0 is an even number... it's not a matter of how the Magic rules "consider" it.

Agreed; even though I entirely agree with Becuzz's point, it doesn't make the question any less valid in theory. Maybe a title such as "How many lands should I play to maximize the chances of hitting with Yennett's ability while still being able to reliably cast it?" would be more accurate.

Run Sensei's Divining Top, "look" into it on the attack trigger and if you don't have an uneven option among the three cards, use the top to draw a card and play the top on resolution of the attack trigger.

I think you're missing few key aspects of Yennett's ability.  Rules text (emphasis mine):

If that card’s converted mana cost is odd, you may cast it without paying its mana cost. Otherwise, draw a card.

This means that lands would go to your hand even if they didn't have 0 CMC because you can't cast lands as spells.  But you're also overlooking the fact that you would still draw the land into your hand even if you miss.  Drawing free lands is still good.  Less good than most free spells, but still good because you can guarantee your land drops.

What should I do to compensate for the lack of mana?

Don't do this.  You should not ever have a lack of mana.  Making sure you hit your land drops is important in Commander so that you can actually play the game.  I would not got below the typical advice of 35-39 lands for a 3 color deck depending on what you your curve looks like.  It's really hard to get to 5 mana with all three colors running much fewer than that.
I would not focus exclusively on 3-mana rocks even though you could get them for free with Yennett.  Mana rocks are at their most useful in accelerating your gameplan.  Once you already have Yennett on the field, does getting a free 3-mana rock really help you more than just guaranteeing your land drop for the turn?  And playing a pair of 2-mana rocks could help you get Yennett out on turn 4 instead of turn 5.
Also keep in mind that if you have a bunch of odd CMC spells, casting cards naturally out of your hand could leave you with awkward amounts of mana left over.  A few quality even CMC spells can fill the gaps and let you get more efficiency out of your mana.  For example, drawing a 2-mana counterspell is still useful, even if your opponents know you have it in your hand.

Yeah. Every card you play will not be one of the free ones with Yennett's ability. If you really wanted to be conscious about it, you could keep your bombs/big dudes in the ODD CMC.

I agree that playing 2-mana rocks would be beneficial, but assuming a proper mana base otherwise, mana accelerators at the cost of 3 mana will perfectly suffice for casting a 5-mana commander at turn 4, whereas you'd need quite a number of them to reliably cast it on turn 3 (not factoring in Sol Ring, which should always allow you to do that).


text:
Hideaway - do I get to put cards put on the bottom of my library in any order?
With Spinerock Knoll, when it enters, hideaway activates, however, it doesn't specify whether I put the cards on the bottom of my library in any order, or in a random order. Which one is it? This is relevant for Grenzo, Dungeon Warden EDH.

Yes, you choose the order:

702.74. Hideaway

702.74a Hideaway represents a static ability and a triggered ability. “Hideaway” means “This permanent enters the battlefield tapped” and “When this permanent enters the battlefield, look at the top four cards of your library. Exile one of them face down and put the rest on the bottom of your library in any order. The exiled card gains ‘Any player who has controlled the permanent that exiled this card may look at this card in the exile zone.’”

Note that the italics text written on  Spinerock Knoll is just reminder text, it is not the actual rules for Hideaway.

207.2. The text box may also contain italicized text that has no game function.

207.2a Reminder text is italicized text within parentheses that summarizes a rule that applies to that card. It usually appears on the same line as the ability it’s relevant to, but it may appear on its own line if it applies to an aspect of the card other than an ability.

So that text is simply a summary of what Hideaway means, but it is not intended to be an actual rule you follow. The full rule for Hideaway is what is defined in 702.74.

Yes, you may choose the order:

401.4. If an effect puts two or more cards in a specific position in a library at the same time, the owner of those cards may arrange them in any order. That library’s owner doesn’t reveal the order in which the cards go into the library.


text:
Quickened Seize the Day
On my opponent's turn, during main phase 2, I Quicken a Seize the Day. My opponent gets an additional combat phase, correct? One of my friends is convinced that the caster of the Seize the Day "controls" the extra combat, and so can attack on an opponents turn, but I'm pretty sure this is not the case. Clarification?

One of the rulings on Seize the Day addresses this situation:

If you somehow cast Seize the Day during an opponent’s main phase, that opponent attacks during the resulting combat phase, since it’s still that player’s turn. If you somehow cast it not during a main phase at all, all that happens is you untap the target creature.

The wording of Seize the Day is pretty straightforward:

Untap target creature. After this main phase, there is an additional combat phase followed by an additional main phase.

It just says "there is another combat phase", not "you get another combat phase". There is no concept of "owning" a phase, other than whose turn it's on.
https://mtg.gamepedia.com/Combat_phase

506.2. During the combat phase, the active player is the attacking player; creatures that player controls may attack. During the combat phase of a two-player game, the nonactive player is the defending player; that player and planeswalkers they control may be attacked.

If you cast it on your opponent's turn, then your opponent is the active player, and so they are the attacking player.

That should work, as it was given flash by quicken and does not state that you get an extra combat phase. It simply states there is another combat phase.

"That should work" is probably not the right response to lead with, as the querent isn't seeking ot understand whether it would work, but whether A or B happens.


text:
Ixalan's Binding and Ashiok, Nightmare Muse
I cast Ixalan's Binding, and its etb targets and exiles my opponent's Rampaging Baloths. Later in the game I use the -7 on Ashiok, Nightmare Muse, choosing to cast my opponent's exiled Rampaging Baloths.
Can my opponent now cast another copy of Rampaging Baloths, since the previously exiled copy is no longer under Binding, or can they still not cast cards named Rampaging Baloths?

From a ruling on Gatherer:

If there is no exiled card (perhaps because the exiled permanent was a
  token or was a commander that moved to the command zone in the
  Commander variant), Ixalan’s Binding won’t stop players from casting
  spells.

If you cast the card from exile using Ashiok then there is no more exiled card.  Therefore, your opponent can cast whatever they want, including Rampaging Baloths.

This ruling could still leave some players unsure; as it refers to what happens in situations when there never was any exiled card. In the OP's case, there actually was an exiled card, it just stopped being exiled.

An example being in the case of tricks like with Fiend Hunter, sacrificing it and causing it's leave-the-battlefield to trigger with no cards in exile.

They will now be able to cast Rampaging Baloths again.
Ixalan's Binding has linked abilities on it; and in order for the second ability to do anything, the card exiled by the first ability must still be in exile.

Linked Abilities

607.2a If an object has an activated or triggered ability printed on it that instructs a player to exile one or more cards and an ability printed on it that refers either to “the exiled cards” or to cards “exiled with [this object],” these abilities are linked. The second ability refers only to cards in the exile zone that were put there as a result of an instruction to exile them in the first ability.


text:
What is the most mana that can be produced by tapping a single permanent in future standard?
After Theros: Beyond Death is added to standard, what will be the most mana a permanent can be tapped to produce?
Rules:

Current standard sets and Theros: Beyond Death are legal
No infinite combos of any sort. This INCLUDES loops that could be repeated turn after turn
Your deck must contain exactly 60 cards
You have set-up time limited only by decking yourself (53 turns, less or more with shenanigans)
You may not use your opponent's deck at all. For the purposes of this challenge, your opponent does not have a deck.
Mirror March is a card that, depending on luck, can technically get any number of mana produced. This came up when I asked a friend about this challenge, but I forgot to put it in the rules before. Any card that creates an "arbitrarily large" amount is ruled out for this challenge (Apologies Arcanist Lupus, although thanks for reminding me)

Answer with the number and the set-up required to do so. After I get a few answers, if mine is still the largest, I will post it. (Hint: I can't express it as a straight number)

Can we assume that the deck is stacked?

Yes. You draw cards in whatever order is optimal for you.

Don't feel obligated to give a turn by turn

Also, can you list the set range for current standard in the question, just so that we can keep track of the exact rules for this question in the future?

Can our solution involve repeating the same activated ability turn after turn? Not sure if this violates rule 2, as it is still limited by the number of turns until you run out of cards.

It does, unfortunately, violate rule #2. That is why it is in place.

OK, now I'm not sure if my answer follows the rules. It has finite loops that are repeated turn after turn but limited by the number of cards in the library. Surely an activated ability like Aetherfox mentioned would be just as OK. Otherwise, that entirely rules out most activated abilities, which can often be activated every turn.

I think I must have misunderstood your answer then, yes, I would determine something like that to be "banned" from this. An activated ability would only be ruled out if it is USED in a way that violates the rules. For example, my friend used a solution that involved both Ral and Expansion, but that was legal, as he didn't use them in an infinite combo. The same logic applies to "loops."

In that case, I don't think I understand the rules. What kinds of loops are allowed?

Any effect, card, or combination of cards that could simply be used turn after turn to generate value is not allowed. Example: Tamiyo, Collector of Tales, but you just plus it then minus it every turn, extending setup simply to use the ability over and over again. Tamiyo is considered a one card loop

Sorry, are you saying that any card or ability that can be used more than once is not allowed?

Let us continue this discussion in chat.

At least 3^(2^^^^39)
Decklist:

4x Nyxbloom Ancient
2x Thousand-Year Storm
1x Repeated Reverberation
1x Narset's Reversal
1x Quasiduplicate
1x Dance of the Manse
1x Wilderness Reclamation
1x Teferi, Time Raveler
1x Awakening of Vitu-Ghazi
4x Izzet Guildgate
4x Simic Guildgate
4x Boros Guildgate
4x Swiftwater Cliffs
4x Temple of Epiphany
4x Azorius Guildgate
3x Gruul Guildgate

Setup for combo
The setup involves the following:

Drawing cards until we have Wilderness Reclamation and 7 other cards in hand, and discarding Wilderness Reclamation to hand size.
Drawing cards and playing lands until we have drawn and played 4 Izzet Guildgate, 3 Simic Guildgate, and 1 Boros Guildgate and all other non-land permanent cards, and drawn all of the instants and sorceries.
Casting Dance of the Manse with X=6 targeting Wilderness Reclamation to put it onto the battlefield as a 4/4 creature.
Casting Awakening of Vitu-Ghazi targeting a

This takes 14 turns to draw all of those cards, and we can finish playing them earlier than that.
Combo
We have no way to untap lands, so we can construct a "loop" that is ultimately limited by the mana that can be generated by the lands on the battlefield.
A loop iteration is as follows:

Cast Repeated Reverberation. Allow all Thousand-Year Storm triggers, previous Repeated Reverberation triggers, and copies to resolve, but leave the original spell on the stack. This copies the next spell to be cast a number of times equal to 2 * (2 * storm count + previous RR triggers). This costs {2}{R}{R}.
Cast Narset's Reversal targeting the Repeated Reverberation card. By now, the storm count is at least 1, so both of the Thousand-Year Storm triggers make at least 2 total copies of Narset's Reversal. Have 1 copy target the Repeated Reverberations card, and the other target the original Narset's Reversal card. The result is that we get 1 more copy of Repeated Reverberation, and all cards on the stack return to the hand. This costs {U}{U}.
Allow all of the copies of Repeated Reverberation to resolve.

This loop iteration costs 6 mana in some combination of {U} and {R}, increases the storm count by 2, and makes twices as many of the next spell to be cast. For the loop, we tap a land for {U} and a land for {R} (both modified by Nyxbloom Ancients), and repeat this loop until we have 5 mana remaining. We spend that 5 mana to cast Quasiduplicate targeting Nyxbloom Ancient, allow all of the copies to resolve, and cast Narset's Reversal to return both cards to the hand. Then we tap another two lands for much more mana, and repeat the process.
When performing this loop on a full set of untapped lands, when spending the last mana on Quasiduplicate, target the awakened Izzet Guildgate instead of Nyxbloom Ancient. When performing the loop during the main phase, target the animated Wilderness Reclamation instead
Then, in a full turn, we do the following:

Activate Teferi's first loyalty ability so that we can cast sorcery spells at instant speed.
Perform the loop described above, copying Nyxbloom Ancient, and with the last lands, Wilderness Reclamation
For each Wilderness Reclamation trigger, perform the same loop, copying Nyxbloom Ancient, and with the last lands, Izzet Guildgate

In the long run, the doubling from Repeated Reverberation far outpaces the storm copies from Thousand-Year Storm, so we can approximately say that each loop iteration doubles the number of copies 8 times. So, after we tap a pair of lands for M =  2 * 3^N mana, where N is the number of copies of Nyxbloom Ancient on the battlefield, we can make approximately 2^((M-5)/6) copies of Nyxbloom Ancient. In the longer run, the multiplicative factors don't even matter that much, so we can say that the number of Nyxbloom Ancients we get from tapping one land is about 2^(3^N), where N is the number we previously had.
2^(3^N) is much greater than 2^(2^N), so we can say that every time we untap our lands, we get more than 2^...^K Nyxbloom Ancients and Izzet Guildgates or Wilderness Reclamations, where K is the number of Nyxbloom Ancients we untapped with and the number of 2s is equal to the number of Izzet Guildgates we untapped with. In Knuth's up-arrow notation, this is much greater than 2^^N, where N is the number of Izzet Guildgates. So, each turn we get 2^^N Wilderness Reclamations and repeat that process in response to each Wilderness Reclamation trigger. That gives us 2^^^(2^^N) Izzet Guildgates.
We repeat that process for the 39 turns it takes to draw the rest of the library, getting more than 2^^^^39 Guildgates, Nyxbloom Ancients, and Wilderness Reclamations. In the end, we tap the last land for more than 3^(2^^^^39) mana.

This cannot go infinite because every land in the decklist unconditionally enters the battlefield tapped, and we can only untap lands during the untap step and a bounded number of times each end step because of Wilderness Reclamation. So, creating more lands can only increase the amount of mana available the next time we untap lands, and the number of times we can untap lands each turn is bounded.

Credit to Arcanist Lupus's answer for the idea of using Wilderness Reclamation, and using Dance of the Manse to be able to copy it with Quasiduplicate.

I think that you're right that Wilderness Reclamation is worth duplicating, but Thousand-Year Storm is not.  Why is Folio of Fancies in your decklist?  I don't see you using it.  Also, why 4 copies of Nyxbloom Ancient?  Wouldn't it be more efficient to use the mana spent playing the three extra copies on duplicating it?

The loop I described uses 10 cards. The Folio is there so that I can keep them all in my hand every turn. And the 4 Nyxbloom Ancients are in there to ensure that I can do a full loop iteration the first time around. They're not strictly necessary but they make the algorithm simpler. I'm sure you can optimize things here and make that last number 50 or more, but there's already so many approximations that it doesn't really matter. The way I see it, the only real improvement would be to add another layer, increasing the number of up arrows.

Still, looking at it again, the benefits of having several of those cards kind of disappear when doing all of those approximations, so I can definitely edit to make it simpler.

Ah, that makes sense.

I did not think of using Wilderness reclamation at all! My own first solution was only 10369*3^3455, which I am relatively sure is smaller than 3^(2^^^41)

Wouldn't Nyx Lotus get you a lot more mana than a land at the end of all this?

@Gilsham Nyx Lotus actually creates an infinite combo here, because it can be reanimated with Dance of the Manse. The copies you could then make with Quasiduplicate wouldn't be creatures and wouldn't have summoning sickness, so you could loop arbitrarily many times just creating copies of Nyx Lotus and then tapping the copies to make more mana to make more copies. Plus, while that would multiply the end result by a very large number, I think it's essentially negligible in comparison to having nearly the same number in the exponent. The various estimates I made probably had more impact.

well, each of the Nyxbloom Ancient will be adding 3 to green devotion right, so surely that is a pretty significant boost to the 3x mana they each give, in fact it seems like it would make the mana generation of them x4 practically

Each Nyxbloom Ancient adds 3 to the devotion, and then each one multiplies the whole result by 3, which is a lot less than making each one multiply by 4. On further reflection you may be right that it would increase the result meaningfully, however you measure that, but I still can't include it because of the infinite loop it creates.

Pretty big
I'm not going to do the math on this, and there may be more optimal configurations, but here's the basic idea:
Ingredients
Dance of the Manse
Thousand-Year Storm
Nyxbloom Ancient
Quasiduplicate
Narset's Reversal
Lotus Field
54 Islands
Gameplan
Go second.  Skip the first land play, and discard Thousand-Year Storm  (TYS)
Play 2 Islands
Play Lotus Field, saccing the two Islands
Play 2 land each turn.  Upon playing your 7th land, play Nyxbloom Ancient (NBA).
Turn 9: Play a 6th land (8th land drop, minus Lotus Field sacs)
Tap Lotus Field for 9 W, 1 island for 3 U
Cast Dance of the Manse with X=6, returning TYS to the battlefield as a creature (2W2U remaining)
Cast Quasiduplicate (QD), targeting TYS.  TYS copies QD, the copy targets NBA.
Let the copy resolve, then tap an island for 9 U (1W9U remaining)
Cast Narset's Reversal* (NR) on QD.  The two copies of NR both target NR.  The first will bounce it, and the second will fissile.  The copy of QD created by the copy of NR will create a copy of TYS.
Cast QD again, this time getting 6 copies.  Then cast NR, getting 8 copies which all target NR.
I think you can see where this is going.  For 5 mana, you get to play Quasiduplicate with Buyback, and tick up your storm counter twice.  This is not an infinite combo, because you can only get mana from your lands, and each land can only be tapped once.  But each land generates a ton of mana (3^N, where N is the number of Nyxbloom Ancients in play).
You can repeat this every turn until you run out of cards in your deck (see, still not infinite).  At the very end, tap your Lotus Field for ungodly gobs of mana.
*For our purposes, when Narset's Reversal creates a copy of a spell, that copy will always target the original target.  There's no benefit to switching targets, so I'm not going to bother to mention it in the main description.

Possible Deck Adjustments
I'm not sure what the optimal ratio of Nyxbloom Ancients to Thousand-Year Storms is for targeting with Quasiduplicate is.  It's possible that Nyxbloom Ancient is always the better target, in which case you can drop Dance of the Manse entirely and replace it with a 55th land. I'm pretty sure that that isn't the case, though.
There will be 2 (or 3 if you drop Dance of the Manse) islands left in your hand at the end of the game when you deck yourself.  You can replace those with Firemind Vessels (which will speed up your engine a turn, and give you more mana on the board), but this is a bit of a tricky point in the rules of the challenge, since if you discard a Vessel the way you discarded the Thousand-Year Storm and then reanimate both of them, the deck goes infinite.  Does it disqualify a deck if the deck is capable of going infinite even if you play it in such a way that you never reach that capability?
Obviously, if you cut Dance of the Manse (as mentioned in the first possible adjustment), then the Vessels are safe to include.
You can also replace one of the islands with Dryad of the Ilysian Grove, which will let you play the other island a turn early.
Another possible substitute for the last land is Repeated Reverberation, which combines with Narset's Reversal to be two Quasiduplicates for the cost of [3RRUU].  This is a better rate than 1 QD for 1UUUU, but it means you have to juggle your mana sources between R and U, and each land can only tap for 1 color at a time (even if its a multicolored land, the tripling will copy the color being produced), so I'm not sure it actually works out as more effective.
You can also include a mana dork (such as Paradise Druid as the last card.  These can be copied by Quasiduplicate, but because they don't have haste they can't be used for mana the turn you create them and you still don't go infinite.  I'm pretty sure that you're better off copying Nyxbloom Ancient over a dork every time, though.
Wilderness Reclamation may also be a valuable addition when combined with Teferi, Time Raveler to give Quasiduplicate flash.  You're effectively doubling the number of lands you have, and if you Dance of the Manse it you can start ratcheting up that multiplicative factor.  This isn't an infinite combo because the untap triggers only happen once per turn, but I'm still not sure if the extra untaps are worth giving up copying Nyxbloom Ancient (My gut says yes, but you also lose a turn to pitching Wilderness Reclamation into the graveyard, so it's hard to judge).

Sorry, I should specify in my answer

I think I can use your Wilderness Reclamation idea to massively improve the number in my answer.


text:
Lazav and Clever Impersonator
Lazav, the Multifarious is activated to become a copy of Clever Impersonator, with Caged Sun out, so it doesn't die. It is now simply a 1/1 with the name, "Lazav, the Multifarious," Lazav's ability, and is legendary. I play Mercurial Pretender, choosing to have it enter as a copy of Clever Impersonator, and so I choose to have it enter as a copy of my opponent's Terastodon, with the intent of destroying some stuff.

Does the Mercurial Pretender turned Clever Impersonator turned Terastodon changing to a copy of certain cards "overwrite" the Legendary supertype from Lazav/Clever Impersonator? Pretty sure this is a yes, but just checking.
If it is "entering the battlefield as a copy" of another creature, than wouldn't it have to enter the battlefield to be able to do anything specific to the creature? Specifically, the Mercurial Pretender would have to enter, become Clever Impersonator, and then, being on the battlefield already, would be unable to become Terastodon. Am I correct on this?

Your first question seems to be written under the assumption that you can do this two-step copy thing, and your second question seems to be written under the assumption that that does not work. It makes the question kind of hard to follow.

"choosing to have it enter as a copy of Clever Impersonator" Do you mean a copy of the Lazav that has copied Clever Impersonator? Why are you not simply having it enter as a copy of Terastodon (not spelling)? Is it just for the sake of generating this hypothetical? "Does the Clever Impersonator turned Clone turned Terrastodon" Do you mean Clone turned Clever Impersonator, turned Terastodon? "and then, being on the battlefield already, would be unable to become Terrastodon." It's not clear how this is "doing anything specific" to a creature, or why it couldn't become Terastodon.

Is question just asking whether the legendary supertype is copied?

@Acccumulation First of all, yes i do mean the lazav copying Clever Impersonator, thought that was self evident and just semantics. Seemed unnecessary to specify. Second of all, my apologies on not just having it enter. I used Clone without checking the card, the intention was to use something that entered as a copy of only  something that you control, I am editing the question to use mercurial pretender

The Mercurial Pretender will enter the battlefield as a copy of Terastodon, without Lazav's name, ability, or Legendary supertype.
After Lazav's ability resolves, it is a 0/0 (plus the +1/+1 from Caged Sun) Legendary Shapeshifter Creature named Lazav, the Multifarious with the following text:

You may have [this creature] enter the battlefield as a copy of any nonland permanent on the battlefield.
{X}: Lazav, the Multifarious becomes a copy of target creature card in your graveyard with converted mana cost X, except its name is Lazav, the Multifarious, it's legendary in addition to its other types, and it has this ability.

These characteristics are the result of the copy effect from Lazav's last ability, so they become its new copyable values. This is specified in rule 706.9:

702.9a Some copy effects cause the copy to gain an ability as part of the copying process. This ability becomes part of the copiable values for the copy, along with any other abilities that were copied.

[...]

706.9c Some copy effects modify a characteristic as part of the copying process. The final value(s) for that characteristic becomes part of the copiable values for the copy.

Mercurial Pretender's ability, and Clever Impersonator's, are enters-the-battlefield replacement effects. When applying replacement effects, you follow the process in rules section 616:

616.1. If two or more replacement and/or prevention effects are attempting to modify the way an event affects an object or player, the affected object’s controller (or its owner if it has no controller) or the affected player chooses one to apply, following the steps listed below. If two or more players have to make these choices at the same time, choices are made in APNAP order (see rule 101.4).
[...]

616.1c If any of the replacement and/or prevention effects would cause an object to become a copy of another object as it enters the battlefield, one of them must be chosen. If not, proceed to rule 616.1d.

[...]

616.1e Once the chosen effect has been applied, this process is repeated (taking into account only replacement or prevention effects that would now be applicable) until there are no more left to apply.

So, as the Clone enters the battlefield, you follow that process like this:

Mercurial Pretender's replacement effect applies. You choose to copy the Lazav with the characteristics above. So, the Mercurial Pretender becomes a 0/0 Legendary Shapeshifter Creature named Lazav, the Multifarious with Clever Impersonator's ability and Lazav's activated ability, and the activated ability added by Mercurial Pretender's copy effect.

Lazav's first ability (from Clever Impersonator) applies. You choose to copy Terastadon. So, this Lazav becomes a 9/9 Elephant Creature named Terastodon with Terastodon's ability. This copy effect overwrites all of the creature's characteristics, so it is no longer Legendary or named Lazav and it no longer has Lazav's ability.

No more replacement effects apply, so the Terastodon enters the battlefield and its ability triggers.


text:
Explanation of Selvala, Explorer Returned's ruling?
Selvala, Explorer Returned has the following ruling:

If you activate Selvala's ability while casting a spell, and you discover you can't produce enough mana to pay that spell's costs, the spell is reversed. The spell returns to whatever zone you were casting it from. You may reverse other mana abilities you activated while casting the spell, but Selvala's ability can't be reversed. Whatever mana that ability produced will be in your mana pool and each player will have drawn a card.

I have 2 questions about this ruling.

Does this mean that, in conjunction with rule 121.8,

121.8 If a spell or ability causes a card to be drawn while another spell is being cast, the drawn card is kept face down until that spell becomes cast (see rule 601.2i). While face down, it’s considered to have no characteristics. The same is true with relation to another ability being activated. If an effect allows or instructs a player to reveal the card as it’s being drawn, it’s revealed after the spell becomes cast or the ability becomes activated.

if you are unable to cast the spell, and it returns to your hand, the cards drawn off the parley ability are kept face down... permanently? In my interpretation, even if you were to recast the spell, since it changed zones, it would have no knowledge of it's previous self, and therefore would not allow the cards to be seen. Is this correct?

Why does this happen? What rulings indicate that the spell should be "un-cast," and is this a unique case? Or is there a precedent with another ruling that does the same?

Where are you reading 121.8?  The version found here, which should be the most recent version as of this post, includes extra text that mentions casting reversal

I think it's strange that that ability is left without the restriction "play only any time you could play an instant", as other random mana abilities have. It would've saved much rules trouble.

Your rule 121.8 is out of date. The current version says this:

If a spell or ability causes a card to be drawn while another spell is being cast, the drawn card is kept face down until that spell becomes cast (see rule 601.2i) or until the casting process is reversed (see rule 725, “Handling Illegal Actions”). While face down, it’s considered to have no characteristics. The same is true with relation to another ability being activated. If an effect allows or instructs a player to reveal the card as it’s being drawn, it’s revealed after the spell becomes cast or the ability becomes activated.

As for why casting the spell is reversed like that, that is covered in the rules that describe the process for casting a spell. Rule 601.2 says

To cast a spell is to take it from where it is (usually the hand), put it on the stack, and pay its costs, so that it will eventually resolve and have its effect. Casting a spell includes proposal of the spell (rules 601.2a–d) and determination and payment of costs (rules 601.2f–h). To cast a spell, a player follows the steps listed below, in order. A player must be legally allowed to cast the spell to begin this process (see rule 601.3). If a player is unable to comply with the requirements of a step listed below while performing that step, the casting of the spell is illegal ; the game returns to the moment before the casting of that spell was proposed (see rule 725, “Handling Illegal Actions”).

Rule 725.1 says this:

If a player takes an illegal action or starts to take an action but can’t legally complete it, the entire action is reversed and any payments already made are canceled. No abilities trigger and no effects apply as a result of an undone action. If the action was casting a spell, the spell returns to the zone it came from. Each player may also reverse any legal mana abilities that player activated while making the illegal play, unless mana from those abilities or from any triggered mana abilities they caused to trigger was spent on another mana ability that wasn’t reversed. Players may not reverse actions that moved cards to a library, moved cards from a library to any zone other than the stack, caused a library to be shuffled, or caused cards from a library to be revealed.

The quoted section is why Selvala's ability can't be reversed along with the rest of the casting process.


text:
Bruvac: Specifying Mill
With the new card Bruvac, the Grandiloquent, and the reformatting of mill as a keyword, I have a couple questions.
It specifically defines "if an opponent would mill one or more cards." Are they errataing things such as Tome Scour, which is clearly "mill?"
In the case that they are errataing, what about cards that are a little more unclear? For example, Tunnel Vision reads "If it is, that player puts the rest of the revealed cards into his graveyard." Because this does not directly read "puts the top N cards of their library into their graveyard," does that mean this is not "mill," and won't be affected by Bruvac?

We don't know because they haven't announced the erratas yet. Before they do, any answers will just be speculation, and after they do, you can just read the card.

I was afraid that might be the case, thanks for clarifying

Agree with murgatroid. As for speculation, I expect Tome Scour and other clear cases to get errata, while cards like Tunnel Vision will not. While both cards technically put a number of the top lib cards into the gy, Tunnel Vision lets you look at them first, unlike a classic mill. Tunnel Vision will also not trigger Bruvac, only cards that actually use the mill keyword do.

I'll just have to stick with the Persistent Petitioners plan then (reasonably sure that'll get errataed)

This doesn't definitely answer the question, but the release notes for Bruvac do say "If a player is instructed to put a card into their graveyard without using the word 'mill,' Bruvac's ability doesn't apply." and "As a rule of thumb, if an effect instructs a player to look at or reveal a card before it's put into its owner's graveyard, it's unlikely to have received errata."

We know that Thought Scour is receiving errata, and it would be very strange for them to errata some old cards with the new wording without doing it to all old cards (obviously excluding those for which the new wording would be functionally different in any way).

The card says on it "To mil a card, a player puts the top card of their library into the graveyard". That to me makes it look like they are defining the act of moving a card from the top of the library as milling. But as others have said we will have to wait until more information is provided.

It's definitely a keyword action. They have announced that much. And the release notes I quoted make it pretty clear that many but not all of the cards that instruct you to put cards from the top of your library into your graveyard will have errata to use that keyword. The reminder text does not imply that every instance of "put N top cards of your library into your graveyard" is mill. It's like how the reminder text for draw can be "To draw a card, a player puts the top card of their library into their hand", but not every instance of moving cards that way is "draw".

Yes, there's a planned errata to introduce the “mill” keyword to earlier cards.
You can already see it in eleven mill cards reprinted in Jumpstart. For example Thought Scour's Jumpstart print says “Target player mills two cards”; it used to say “Target player puts the top two cards of their library into their graveyard”. Tome Scour uses the same template as Thought Scour except it says “five” instead of “two”, so it will assuredly receive errata to say “Target player mills five cards.”
Eli Shiffrin, the rules manager, tweeted guidelines about what to expect with the new mill errata and says it should arrive in Gatherer on Tuesday (meaning the 23rd of June):

Handy rule of thumb on the upcoming "mill" errata (in Gatherer Tuesday): If you look at the cards and then move those specific cards to the yard, it's not "mill." If you move cards and then look at them (or don't look at them at all), it's probably "mill." #WotCStaff

He confirms as an example that Mind Funeral will not say “mill” but Traumatize will. This makes it fairly clear Tunnel Vision will not receive errata to say “mill”.

It looks like the errata is now in Gatherer, so this answer doesn't need to guess anymore: Traumatize has errata to say "mill", and Tunnel Vision does not.


text:
MTG Arena - casting during opponent's upkeep
In MTG Arena, I keep trying to set a stop after my turn so that I can cast during my opponent's upkeep, but it always appears to skip right into their main phase.
It's been a while since I played Magic, but casting spells during opposing upkeep used to be a mainstay of my play (i.e., tapping a land with icy manipulator during upkeep so that the mana isn't available during the main phase).
Is this no longer something you can do in Magic, and if so, how do I set it up to allow me to do it in MTG Arena? Even with full stops enabled, it appears to breeze right past upkeep.

Can you explain in what way you enabled full stops? Also, at the time you were performing this test, did you have a spell or ability that you could use during the opponent's upkeep?

Well, I've noticed it all of the way around, but I've been testing with a mono blue deck that has plenty of castable cards: merfolk trickster, blink of an eye, etc. I was trying to merfolk trickster a llanowar elf during upkeep to remove the mana production.

Ok, I just figured out how -- you have to shift-control so it shows "Hold Full Control" and it looked like it did it. I had been trying to just hit control to enable it.

So, to be sure I understand, are you saying that what you tried before asking the question was pressing or holding control at some point in the game?


text:
How does Sundering Titan handle snow lands?
My doubt is about Sundering Titan; what happens when it enters and leaves the battlefield if there are snow lands in play? Does it destroy basic lands and snow basic lands and how much of each of them?

Yes, it can destroy both snow-covered basic lands and non-snow-covered basic lands. It's up to Sundering Titan's controller; he or she must choose one land of each basic type (Plains, Island, Swamp, Mountain, Forest) (provided such a land is present on the battlefield). Snow-covered lands have basic land types, so the Titan's controller can choose them as well, but he/she can't destroy e.g. a Mountain and a Snow-covered Mountain during the same effect.

It doesn't even have to be a basic land, just to contain a basic type. A gate for example, which is a nonbasic land, can be picked twice. You pick basic landtypes, not necessarily basic lands.

@Mast, you are claiming gates can be destroyed by Sundering Titan.  That is not the case.  They don't have any basic land types.  Tropical Island is an example of a non-basic land, that has basic land types.

@Mast Gates do not have basic land types, and therefore cannot be chosen for Sundering Titan

@Hackworth If a guild gate has both plains and island as type (Azorius gate), those are both basic land types and it should be able to be destroyed, no?

The fact that it produces W and U doesn't mean it has Plains or Island as subtypes...

@Mast No, gates such as Azorius Guildgate have only the gate subtype. Just because it produces blue and white mana doesnt mean it has the island and/or plains type. What matters is the type line, and for gates it says "Land - Gate". Land is the type, Gate is the only subtype. Sundering Titan can choose lands that have the Plains, Island, Swamp, Mountain, and/or Forest subtype.

"Snow" is a supertype.  If you look at Snow-Covered Plains you will see that it's types are "Snow Basic Land - Plains".  Which means that it counts as a basic land and a plains.
So in the case of Sundering Titan, if there was both a regular Plains card and a Snow Covered Plains out, you could only choose one of them to destroy.
Relevant rules:
From the comprehensive rules glossary

Basic Land Type There are five "basic land types": Plains, Island,
  Swamp, Mountain, and Forest. Each one has a mana ability associated
  with it. See rule 305, "Lands."

And

205.4c Any land with the supertype "basic" is a basic land. Any land that doesn't have this supertype is a nonbasic land, even if it has a
  basic land type.: Cards printed in sets prior to the Eighth Edition
  core set didn't use the word "basic" to indicate a basic land. Cards
  from those sets with the following names are basic lands and have
  received errata in the Oracle card reference accordingly: Forest,
  Island, Mountain, Plains, Swamp, Snow-Covered Forest, Snow-Covered
  Island, Snow-Covered Mountain, Snow-Covered Plains, and Snow-Covered
  Swamp.

And from the rulings on Snow Covered Plains:

“Snow” has no particular meaning or rules associated with it.

Also (emphasis mine)

Since this is a basic land, you may have any number of it in a
  Constructed deck in any format in which the Masters Edition II set,
  the Coldsnap set, or the Ice Age set is legal.

Your link appears to be broken.

Sundering Titan can destroy snow lands because cares about lands having a basic land type (Forest, Plains, Island, Swamp, Mountain), not about those lands having the name "Forest", "Plains", etc.
For each land type (Forest, Plains, Island, Swamp, Mountain), you get to choose a land that has that type to destroy.  Here are some examples of lands that can be chosen as a forest by Sundering Titan:

Forest
Snow-Covered Forest
Temple Garden
Sapseep Forest
Murmuring Bosk
Dryad Arbor
Any saproling creature if Life and Limb is in play


text:
What happens if I blink a transformed double-faced planeswalker?
If I blink Garruk, the Veil-Cursed with something like Skybind, does he come back and instantly die?

Correct, I jumped the gun a bit. Should read the cards first.

When you blink Garruk, the Veil-Cursed, it will return as Garruk Relentless as if you had just cast it.
When a permanent such as a planeswalker leaves the battlefield, it becomes a new object with no relation to its former existence. When it returns, it will enter the battlefield as if you had cast it from your hand, non-transformed and with full loyalty.

400.7. An object that moves from one zone to another becomes a new object with no memory of, or relation to, its previous existence.
121.2. Counters on an object are not retained if that object moves from one zone to another. The counters are not “removed”; they simply cease to exist. See rule 400.7.
209.1. Each planeswalker card has a loyalty number printed in its lower right corner. This indicates its loyalty while it’s not on the battlefield, and it also indicates that the planeswalker enters the battlefield with that many loyalty counters on it.
711.8. A double-faced card enters the battlefield with its front face up by default. [..]


text:
Are there other legal/recognized Roulette variations than just the standard European and American?
Currently, I know there are two main variations of the Casino Roulette layouts, the European (with a Single Zero "0") and the American (with Zero "0" and Double Zero "00"). Roulettes each with their characteristic sequence of numbers, and with their boards with numbers arranged in reading order:

I am not at all sure, but in the past I remember to have played in a Casino, with the double zero layout memorized, just to realize they had a different double zero layout.
Question is: Is there any other "legal" or recognized layout around there? Or the strongest possibility is I was a bit too drunk that night?

Do you remember what country the casino was in?

A google image search will find the various different layouts including a triple zero layout.

This wheel was 0 and 00 adjacent.  the page I found it on  Says it in a casino in Australia.
If you search for 'triple zero roulette'  You will find many images of that layout as well and casinos that use them.


text:
Use Mana twice for the same spell by untapping?
If your commander has an ability to untap card and you use it for untapping Mana, can you use it to pay for 6 Mana while there is only five in your pool.it was Estrid, the Masked untapping an enchanted land.

You don't "tap Mana."  You tap Lands, which produce Mana.

I've rolled back the edit that fixed the conceptual mistakes in the question. Those conceptual mistakes are materially relevant to answers here so that we can guide the querent onto an accurate conception of how the game works. Correcting those mistakes is something to do in an answer, not in an edit. Leaving those present in the question allows us to do this accurately, editing them out is sorta brushing them under the carpet when they need addressing.

Technically, you can't use it to pay 6 Mana when there are 5 in your Mana pool; once you use it, you now have 6 Mana in your Mana pool. I'm not sure whether this is enough of a misconception to be included in an answer.

Yes, you can tap a land for mana again in the same turn if it becomes untapped.
When you can tap a land for mana, it always has some variation of "{T}: Add ..." as its ability, where {T} is the tap symbol. That is an activated ability. Activated abilities always follow the "cost: effect" pattern. Generally, you can activate an activated ability as often as you want, as long as you can pay its cost every time.
When you tap a land for mana, the mana it produces goes into the "mana pool", a temporary storage for your mana. Then, when you untap that land, you can pay the cost of its mana-generating ability again (i.e. tapping the land), and you will get another mana.
That means, if you have 5 lands, tap them for mana, and untap one of the lands, you can tap that land again to get 6 mana total.

602.1. Activated abilities have a cost and an effect. They are written as “[Cost]: [Effect.] [Activation instructions (if any).]”
602.1a The activation cost is everything before the colon (:). An ability’s activation cost must be paid by the player who is activating it.
106.4. When an effect instructs a player to add mana, that mana goes into a player’s mana pool. From there, it can be used to pay costs immediately, or it can stay in the player’s mana pool as unspent mana. Each player’s mana pool empties at the end of each step and phase, and the player is said to lose this mana. [..]

Part of this confusion cames from the fact that most lands don't have the "{T}: Add [whatever] to your mana pool". For a newbie, it is quite easy to play for quite a bit without being aware that this is a implicit text for lands. When teaching MTG to one of my kids, I printed a couple of important notes like this one for lands, or how regeneration, fly and initiative works, or that you could use planeswalker's effects once a turn and put then in the relevant card's sleeves, as "trainers". It is helping her a lot, and she can even clear some doubts from other newbies using them, now.

A minor point of clarification: Lands actually have "mana-abilities" (a specialized subset of "activated abilities", though non-basic lands may have both). MOST of the time this point is irrelevant, but it's the reason you can't [[Stifle]] someone tapping their lands or creatures for mana. See 605.3b.

@SableDreamer That is correct but I wanted to keep the answer simple. Explaining what a mana bility is would not have added anything helpful for the answer, given the basic level of the question.

@T.Sar and another common newbie mistake about the mana pool, believing you tap as part of paying the cost, not before paying the cost to 'float' the mana.

In older editions, the "{T}: Add [relevant color mana] to your mana pool" was explicitly on the land's text, rather than just the big symbol (or whatever it is these days).

It might be worth pointing out that the mana pool empties between steps and phases. So if you have an "untap at start of end step" effect you need to wait until the end step to tap the land for mana, then untap, then tap more land.

@TimB If you read my rules quotations, it says so right there in 106.4

@Hackworth Yes, I meant explicitly pointing it out in your last paragraph before the quote block. i.e. "so long as you do so before the mana pool empties at the end of the step or phase"

First we need to clarify what lands and mana are, because you're getting these mixed up.
The land cards you lay out in front of you are merely Lands. They are not mana or your mana pool. Lands are tapped to acquire mana, which then acts as a currency to pay for spells, as you'd be familiar with. The “mana pool” does not actually physically exist anywhere on the table, it's purely conceptual as a way to think about your mana.(note)
This means when you tap a Forest you get {G} (one green mana), and you spend that {G} to cast a spell. Other cards such as Llanowar Elves can also give you mana, although they are not lands. Some cards such as Cascading Cataracts can give you more than one mana.
You can absolutely untap a land and then tap it again to obtain more mana. In fact this is why cards like Arbor Elf or Blossom Dryad exist: they let you untap a land so that you can tap it again for more mana.
If you had five basic lands (let's say 2 plains, 2 islands, 1 forest) you can tap each of them for mana. Then, yes, you can use Estrid's ability to untap the one that's enchanted, and tap that again for one more mana. You would then have six mana.

Note: The “mana pool” concept no longer gets referenced on new cards, as of Dominaria. Cards printed in and after that set only reference adding mana, but without referring to a mana pool when they do so.

The mana pool has very much not been retired. It just isn't used in wordings any more.

@Hackworth Oh hey, looks like it's still used in the CR. I'll revise that note. Thanks for the comment.

YES, you can.
A land really has the implicit text "Tap: Add [land_specific mana] to your mana pool". So, the cost to activate this mana-producing ability is to TAP the land.
Other mana sources require tapping (for example, Llanowar Elves, or Mox artifacts), others require sacrificing (for example, the "Black Lotus" from the alpha/beta edition), and so on. As long as you can meet the cost, you may activate the ability.
So, if you activate your lands to add the mana to your pool, THEN activate the loyalty ability from the planeswalker to untap all the enchanted permanents you control, and some of these enchanted permanents happen to be lands, sure, you can tap them again to add their mana to your pool once more.
However, you need to keep in mind that you may only activate a Planeswalker's Loyalty ability as a Sorcery (and only ONE such ability par turn and per planeswalker. Also, if the number on the ability is positive, you gain that much loyalty on that planeswalker, but if the number is negative, you need to have enough loyalty on the planeswalker to "pay" for the ability.)
This means that you CANNOT use the planeswalker's loyalty ability WHILE casting your spell, and getting the mana. That's not a mana-providing ability, it cannot be played as a fast effect, only as a sorcery. You need to do so BEFORE you cast your spell.

You can't use any ability, fast or not, while casting a spell. Casting a spell consists of paying the cost and putting the effect on the stack, all instantaneously.

To clarify that, you cannot activate most activated abilities while casting a spell, but you can activate mana abilities, like the ones on lands.


text:
Playing cards without a check in Pathfinder
Somewhere in the Pathfinder ACG rule book, I remembered reading, that for each check, any player may use one card of each of the different card types, meaning only a single Spell card, Weapon card, Item card, etc.
I don't remember finding in the rules, if I am able to use cards when I am 'between checks'.
For example, if I have 2 Cure cards in my hand, can I use both cards, BEFORE my first free exploration?

Yes playing both cards outside of an encounter are allowed. From the Pathfinder Skull and Shackles rulebook (and probably the others as well as the rule hasn't changed) on page 9:

Playing Cards
Anyone can play a card whenever the card allows it. Playing a card
  means using a power on that card by revealing, displaying, discarding,
  recharging, burying, or banishing that card or by performing another
  action specified by that card.


text:
Can I play a card to move out of a player's property and avoid paying rent?
When you land on someone’s space and they own that space, instead of paying rent can you just use a card that says move to any space. Like you can move somewhere a else while it is still your turn.

The only cards which move you to another space are Chance and Community Chest cards. All of those cards take effect immediately upon drawing them and they are discarded, aside from the Get out of jail free cards (which you keep). Thus, it is impossible for you to have a card that moves you to another space, unless you are in jail - and you are not paying rent to anyone in jail...

Have to admit, that would make it a pretty cool house-rule to let those move cards be kept and played at any time, although it removes the downside because if someone has a hotel on Boardwalk all you have to do is choose to not move there...

@corsiKa, Would make more sense to be able to use them before rolling. In the scenario you presented, the player did land on Boardwalk and thus owes rent.

@ikegami I think that'd be a good way around that problem, yeah.

@corsika this would reduce the dangers of cards that move the player to an expensive property when they are short on cash and in general that goes against the game and is a type of house rule that makes the game last for hours.

@JoeW As I said, "it removes the downside because if someone has a hotel..." - yes, I agree if not properly developed it's the kind that make the game drag out. Although most of those house rules exist specifically so no one can lose because little kids don't like to lose. Who would have thought having little kids play a game designed to illustrate the devastating effects of capitalism would cause those kids to cry?!


text:
Does Humility prevent Manifested creatures from being turned face-up?
Related to Does Humility stop all creatures from being able to morph?: The question specifies morphs, but I wondered if this was true for Manifest as well? As Manifest is not an ability on the card, does Humility prevent it from being turned face up for its mana cost?

You can still turn a Manifested card face up even if it would lose all abilities from a card like Humility.
This is actually called out explicitly in a ruling on Manifest cards:

Unlike a face-down creature that was cast using the morph ability, a manifested creature may still be turned face up after it loses its abilities if it’s a creature card. (2014-11-24)

This is because the thing that lets you turn a Manifested card face-up is not an ability on the card itself that it would lose. Instead, the thing that lets you turn a Manifested card face-up is the Manifest rules themselves, which just say you can turn it face-up for its mana cost any time you have priority.

The answer is obviously very good; but there are several details which have been underestimated here, and should be rediscussed. For this, in my opinion there are many more details in the duplicate question "Why can a manifested creature be turned face up if it loses all it's abilities but a morphed card can't? [Duplicate]", especially in the comments at the bottom of the post.

Otherwise, it is not clear how a 2/2 vanilla may "lose all its abilities": how can it do it, if it is true that a vanilla has no abilities by definition? It is clear that something here is missing; there are precise connections, which are unclear here, between the moment a card loses its abilities, and the moment it is unmorphed.


text:
Which commanders benefit from being in the Command Zone other than Oloro?
Oloro, Ageless Ascetic, gains its controller 2 life every turn it's in the command zone. Are there ANY other possible commanders that have abilities or benefits of being in the command zone or is he really the only one?

Related: In what ways does a Commander in their Command zone influence the game?

15 possible commanders interact with the command zone in an unusual way. 8 commanders have abilities that function in the command zone. All of them were created for the preconstructed commander decks or released in commander specific sets. They even eventually created the "Eminence" ability word for  abilities that function in the command zone.

Arahbo, Roar of the World has the ability "Eminence — At the beginning of combat on your turn, if Arahbo, Roar of the World is in the command zone or on the battlefield, another target Cat you control gets +3/+3 until end of turn." giving you a free boost to one attacker each turn.
Derevi, Empyrial Tactician has the ability ": Put Derevi onto the battlefield from the command zone." This lets you play the commander without casting it, so it cannot be countered as a spell (but as an activated ability, with Stifle) and avoids the commander tax* so it will always cost 4 mana.
Edgar Markov has the ability "Eminence — Whenever you cast another Vampire spell, if Edgar Markov is in the command zone or on the battlefield, create a 1/1 black Vampire creature token." giving you creature tokens for playing tribal vampires.
Inalla, Archmage Ritualist has the ability "Eminence — Whenever another nontoken Wizard enters the battlefield under your control, if Inalla, Archmage Ritualist is in the command zone or on the battlefield, you may pay . If you do, create a token that's a copy of that Wizard. The token gains haste. Exile it at the beginning of the next end step." allowing you to get a token copy of your wizards for one turn, potentially triggering ETB effects or letting you use their tap abilities earlier.
Liesa, Shroud of Dusk - Liesa has the ability "Rather than pay  for each previous time you've cast this spell from the command zone this game, pay 2 life that many times." forcing you to use life instead of mana for the command tax when casting her.
Oloro, Ageless Ascetic - Oloro has the ability "At the beginning of your upkeep, if Oloro, Ageless Ascetic is in the command zone, you gain 2 life."
The Ur-Dragon - The Ur-Dragon has the ability "Eminence — As long as The Ur-Dragon is in the command zone or on the battlefield, other Dragon spells you cast cost  less to cast." effectively making him a Dragonlord's Servant on or off the field.
Yuriko, the Tiger's Shadow - Yuriko has "Commander Ninjutsu " functioning like ninjutsu from Kamigawa. This lets you bounce an unblocked attacker to put Yuriko into play. Like Derevi, this is not casting, so you will not pay commander tax.

Currently 7 other possible commanders interact with the command zone in an unusual way, but not through an ability that works there. None of them require the creature be your commander :

Captain Vargus Wrath - gives +1/+1 to all pirates (himself included) when he attacks for each time you have cast your commander from the command zone. He doesn't need to be your commander for this effect.
Jeska, Thrice Reborn - her starting loyalty is the number of times you have cast your commander this game. She does not need to be your commander for this effect.
Jirina Kudro - creates tokens when she enters for every time you have cast your commander this game. She does not need to be your commander for this effect.
Reyhan, Last of the Abzan lets you put counters on creatures when another creature with counters dies or when it is returned to the command zone.
Tevesh Szat, Doom of Fools - Puts all commanders onto the battlefield under your control, taking them from other player's fields or their command zones.
Henzie "Toolbox" Torre reduces Blitz costs by  for each time you have cast your commander from the command zone. He doesn't need to be your commander for this effect.
The Swarmlord comes in with 2 +1/+1 counters for each time you have cast your commander from the command zone. He doesn't need to be your commander for this effect.

There are also many other cards that interact with the command zone but can't be commanders, most with effects that scale based on the number of times you have cast your commander or move your commander out of the command zone into your hand or onto the battlefield.

* This is the cost increase for casting your commander from the command zone again every time it is sent back to the command zone.
With a recent rules change, commanders die like normal creatures before being moved to the command zone. This means Reyhan's ability handles dying commanders like it handles all other creatures. Commanders being sent to the hand or deck will still work as before, those replacement effects are unchanged. A commander being exiled will not trigger Reyhan anymore, since they are sent to exile as normal and then they are no longer a creature you control when they are sent to the command zone. This is still true of Skullbriar, the Walking Grave who, while he keeps his counters moving between zones, is not a creature you control when in the graveyard or exile, and won't trigger the effect a second time.

You say that Reyhan's ability no longer interacts with the command zone, but it hasn't been errata'd, and there are still ways to put creatures into the command zone from the battlefield, so I don't think that's right.

^ Any commander that's about to be bounced or tucked could instead go straight to the command zone from the battlefield.

@murgatroid99 You two are right, and I have a few other errors there. Editing.

In addition to that, the card Leadership Vacuum exists.

@murgatroid99 True, as does Road of Return, Netherborn Altar and Hellkite Courser I will remove that section where I lay out all non-commanders and just leave the search link.

I'm not going to write a whole new answer for it, but what would you think about adding Grist, the Hunger Tide to this answer?

@PhilipKendall Grist is debatable - she doesn't interact with the command zone in any special way, or give you some benefit or ability while she is in there, but can be a commander because of the CDA making her a creature during deck construction and allowing her to be chosen as commander.

I'd say a card without the Creature type (or explicitly being allowed to be a Commander) being in the command zone at all is an unusual interaction with the command zone :-), but your call.

@PhilipKendall Truth be told, part of the reason this list is those 13 and not just the first 8 is because of the simple scryfall query to find cards that can be a commander and specifically reference the command zone (is commander and full oracle text includes "command zone")

This answer seems to be getting well beyond the actual scope of the question.

There are others, such as Derevi, Empyrial Tactician or any creature with "eminence".
Try the following search:
https://scryfall.com/search?q=o%3A%22command+zone%22+t%3Alegendary&unique=cards&as=grid&order=name