This is the main page:
http://www.swordsorcery.com/gammaworld/
Gamma World is basically a post-Apocalypse game, set well after the world has pretty much been wiped out. The newest edition (6th) uses d20 Modern rules as it's base (which mostly fits the scene). I tend to use a mix of 1st, 4th and 5th Ed as well as the 6th Ed stuff.
The PCs have a choice of a wide range of starting mutations and abilities. Again, I mix in more psionics than are present in the 6th Ed book. And in the current game I am running, the PCs are all stone-age peoples struggling to understand their world.
I can quite recommend it.
Cheers mate,
TGHO