GURPS Ground Zero - A large collection of realism-oriented rules options, mostly focused on combat. - http://www.kolumbus.fi/~kj2275/gurps/
MyGURPS - House rules, resources, articles, GM aids, and reviews. - http://www.mygurps.com/
Richard's Cavalcade of Whimsy - GURPS variants and random monster abilities. - http://www.cavalcadeofwhimsy.com/rpg/rpg_stuff.htm
Unlimited Mana - Originally printed in issue 9 of Pyramid magazine, this GURPS Magic variant is one of the most popular in the history of the game. Includes a PDF version of the Calamity Table. - http://www.io.com/~sjohn/unlimited-mana.htm
Self-Esteem and Repartee in GURPS Swashbucklers - Steffan O'Sullivan's adaption of an innovative concept from Lace & Steel, neatly ported to GURPS. - http://www.sjgames.com/gurps/articles/repartee.html
Gunmetal Blue: A Truly Badass GURPS Page - A page devoted to using GURPS as the ultimate system for cinematic action roleplaying, recreating the exploits found in anything from Terminator 2 to Hard Boiled. - http://www.io.com/~sjohn/action.htm
GURPS Sex - A mini-netbook discussing everybody's favorite topic, by Tim and Christine Morgan, who presumably playtested it. - http://www.eskimo.com/~vecna/gurps_sex.html
Converting A Campaign To GURPS - An article on making the move to GURPS from another system, using the author's Mechwarrior conversion as an example. From Roleplayer 19. - http://www.sjgames.com/gurps/Roleplayer/Roleplayer19/CampaignConversion.html
GURPS Gladiators - A standalone combat boardgame using the GURPS advanced combat rules. Also useful as a GURPS teaching tool. - http://www.io.com/~sjohn/glads.htm