Determining what set I have from a bag of lego pieces? I have a bag of lego pieces that make a set of something but I don't have a box or instructions.  Is there anyway to determine what set it is so I can download/buy an instruction sheet for it...or at least know what I am building?  

I found a part piece with a number on it and found out what it was...Spiderman's doc ock ambush.
I see you already found the answer, but in general, you can use Rebrickable. After registering, in the My RB menu you can upload the parts you have and the site will tell you which sets you can build out of them. It will even show partial matches, listing the bricks you still need to build them.