The official website for Gamma World d20

The homepage of a Gamma World webring

The homepage of another Gamma World web ring

The homepage of yet another Gamma World web ring

Spaceports, a Gamma World website

WotC official Gamma World website (not much there)

This Gamma World website has a lot of material, including a copy of the 3rd edition rules!

Wolfy maintains a Gamma World website with a few resources

The Apocalyptic Post is a Gamma World semi-monthly publication

Tormentor's Virtual Wasteland, a website with some resources

Post-Apocalyptic Media is a website devoted to the apocalyptic mythology, not Gamma World in specific

Wyldfyre has a website with some resources for Gamma World 4th edition

The Gamma World Resource Page is just that, full of GW goodness

GammaHammer maintains a website that has a lot of info for GW 3rd edition as well as for Omega World, the d20 version