GURPS

Blue Room GURPS - A large collection of "classic era" GURPS articles, adventures, design notes, and product ratings by S. John Ross, author of GURPS Warehouse 23 and other supplements. Includes the Unlimited Mana rules.
http://www.io.com/~sjohn/gurps.htm

GURPS: Generic Universal Roleplaying System - The official Steve Jackson Games page for GURPS. Features news on new and upcoming releases, and links to the massive SJ Games library of online resources.
http://www.sjgames.com/gurps

GURPS Character Repository - A collection of characters, adventures, house rules and links. By SD Anderson.
http://www.geocities.com/TimesSquare/Dungeon/5264/

Brandon Cope - Vehicles, characters and campaign settings for GURPS and Traveller.
http://www.geocities.com/copeab/

Ebon Tower Fantasy Gaming - A general-purpose fantasy gaming webzine with a strong GURPS slant. Templates, revised rules, and a campaign setting.
http://www.telisphere.com/~davidl

GURPSnet - Mailing list on GURPS with archive of vehicles, gear, characters and adventures.
http://gurpsnet.sjgames.com/

GURPSWiki - Articles on general GURPS topics.
http://www.gurpswiki.net/

MOD-RPG - New and altered rules for GURPS campaigns.
http://modrpg.lab.pro.br/

Rare GURPS Items - Incanus' collection of scraps, tidbits, articles, and variants - dozens of GURPS items, including original work and articles culled from Usenet and sundry sources.
http://www.thecabal.org/gurps/rareitems/

GURPS Stuff by Scott Maykrantz - A collection of articles, rules, and notions from the keyboard of Scott Paul Maykrantz, author of GURPS Creatures of the Night.
http://www.omnology.com/om-gurps.html

DM's GURPS Page - Articles, including technical discussions, rules articles, adventures and seeds, and the Amber Nebula SF campaign.
http://www.dangermouse.net/gurps/

GURPS Frequently Asked Questions - The official FAQ maintained by Steve Jackson Games.
http://www.sjgames.com/gurps/faq/