A very good one I used to play (years ago) is GURPS. GURPS basically takes the core of D&D, the expands on them infinitely. GURPS is like a base class of something, which each different GM uses as a base for his/her game. I played 2 medieval and 1 futuristic GURPS games, and they owned hardcore.
There was also another one with giant robots (like...kinda real. Little metal die-cast robots, it was pwn). Anyone know the name of it?