Chess engines for analysing games Up untill now, I have used Rybka for analysis of my games.
What other chess engines have you used, and how helpful are they?

I am not interested in a list of chess engines with their respective websites, the name of an engine you have just heard of. I want a fact-based review from someone who has used the engine mentioned in their answer for a long time, and found and helpful.

What I am looking for in the mentioned engine is to find a way to calculate the accuracy of the game:


How often the player was making good moves.
How often the advantage in the game was changing 


Previously I saw such options in Rybka, but in my current version Rybka v4, I cannot see this option as available.

Many thanks to @ETD for a helping hand in giving a direction on what exactly I want in an answer.
I use Stockfish, and have found it useful for analysis largely because of its multipv support (I switched from Crafty a couple of years ago), and it being free. I feel it lacks EGTB support, so it's not up to par with Rybka in that respect, but haven't felt willing to shell out the money for Rybka yet, just for that feature. It sounds like it also might do a better job of reporting forced draws, but I haven't confirmed that myself.

I agree with Ed, you should give guidance on what you want to hear in your answers. Rybka is a fine analysis engine.