Must be dodgy sites you visit! :-)
Anyway, a "crack" is often a term used to designate a piece of software that can be run to get around registration - ie it "cracks" the code which limits the software to a demo-mode or to time limitations.
Maybe the same for games - if you have a trial, you can get a crack to make it a full game. Or perhaps allows you to get to the next level without finishing the current one... can't be specific as I don't play games (nor indeed look for cracks!).