Gamma World
Gamma World is a post-apocalyptic RPG created by TSR and set some 300 or so years in the future, after a global catastrophe. The specifics of the end of the world have changed 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 such as the C.E.R.N's Large Hadron Collider ripping apart the fabric of reality and letting no end of weird sci-fi shit spill in.
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. This could result in pure comedy as players use unknown devices like a hair dryer in the false belief it is some sort of Ancient's death ray gun. Initially three levels of technology existed for different cultures: primitive, Medieval-level (or Mad Max-level if you prefer) and high-tech for cultures who preserved the knowledge of the Ancients.
In addition to living in a world full of crazed creatures with weird powers, there are also equally crazed "Cryptic Alliances". These societies exist to make the game world even more awesome, including the Radioactivists, who worship "The Glow"; The Red Death, a biker gang who exist purely to kill you; and the army of Bonapartists led by a mutant sentient bear called Napoleon, who will probably want to kill you too.
There is a d20 Modern supplement covering it in the book d20 Future (is it really gamma world?)_. It is exactly as awesome as it sounds. Jonathan Tweet also penned an article in Dungeon Magazine(2002, issue 94) "Omega world" which despite being just a few pages long was a heck of a lot better than the sixth edition crapped out by White Wolf. White Wolf succeeded in removing anything possibly fun and costing you more money with the release several turgid books needed to play the game. There is also, oddly enough, an adaptation of Gamma World to the Star Frontiers game, with reworked statistics for Star Frontiers, called Gamma Dawn.
The seventh edition saw the introduction of cards which bring in a collectable trading card game component. This sadly ruins what would have been an awesome edition and removed the vile after-taste of the lack luster White Wolf edition. WOTC decided that selling a near empty box without even any dice in it was a good idea.
In closing second edition is probably one of the most awesome editions as not only do you get a lot of useful material and a pretty map to look at, but best of all Pittsburgh gets nuked. Also it has a Cryptic Alliance called the Knights of Genetic Purity, basically human redneck racists who only let Pure-Strain Humans live.