Rules and Variants

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

D&D 3.5 to GURPS 4th Edition
Character conversion formulas by Bruce Grubb.
http://members.aol.com/BruceG6069/ADnD_to_GURPS.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 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

GURPS Ground Zero
A large collection of realism-oriented rules options, mostly focused on combat.
http://www.kolumbus.fi/~kj2275/gurps/

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

Matt's GURPS page
Variant game mechanics and rules simplifications.
http://www.geocities.com/geowars/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

Self-Esteem and Repartee in GURPS Swashbucklers
Steffan O'Sullivan's adaption of an innovative concept from Lace and Steel, neatly ported to GURPS.
http://www.sjgames.com/gurps/articles/repartee.html

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