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

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

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

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

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

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

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 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

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/