Most game systems can be played in a variety of genres. D20 is mostly fantasy but they have lasers in there and there was a Star Wars RPG on that system. White Wolf is mostly modern/urban fantasy but they came out with Aeon and Exalted. Palladium not only has multiple genres but have a mashup called Rifts.
hehe - I havent played D&D sense wizards of the coast raped it .. >_<
I prefer champs for superheros and Rolemaster for fantasy and sci fi-
but ive also played Top secret SI and the original Top secret, as well as D&D up to second ed - the black books - some early gamma world and Mechwarrior.. ( the rpg and table top combat battle tech ) good pages...