G.U.R.P.S. => The Generic Universal Role Playing System. is perhaps one of the most beloved and one of the most despised systems ever published.

This little jewl (no bias here eh..) from Steve Jackson Games. Is in my opinion the swiss army knife of role playing games. It consits of a main set of rules that create a simple set of system mechanics designed to have additional rules and information attatched to them as desired. GURPS does not on its own attempt to represent any particuler feel or genre. It does however atempt to be realistic, and provide basic information for a veriety of settings from medeivil magic, to scifi and psionics.

Additional books covering everything from high technology and magic to martial arts and super powers. Also there are "world books" detiailing certian settings for those who would find them entertaining or just don't have time to come up with there own.

GURPS also has serveral liscensed suplements converting other populer games and settings.

GURPS trully is equiped to handle any genre competently
Fantasy
Super Heros
SciFi
Comedic
Etc