Gamma World
Gamma World is a post-apocalyptic RPG created by TSR set some 300 or so years in the future, after a global catastrophe. The specifics of the end of the world have chanced with advancing technology- in early editions, nuclear war was responsible; in later versions the catalyst was nanotechnology, genetic engineering, or other extrapolations of current technology.
The common player races include humans, mutants, sentient animals or plants, and androids.
Like most post-apocalyptic settings, the people of Gamma World had to scrounge for high-tech devices like guns, since the knowledge to create them had been lost in the cataclysm.
There is a d20 Modern supplement covering it in the book d20 Future. It is exactly as awesome as it sounds.