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.
D&D 3.5 to GURPS 4th Edition: Character conversion formulas by Bruce Grubb.
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.
GURPS Gladiators: A standalone combat boardgame using the GURPS advanced combat rules. Also useful as a GURPS teaching tool.
GURPS Ground Zero: A large collection of realism-oriented rules options, mostly focused on combat.
GURPS Sex: A mini-netbook discussing everybody's favorite topic, by Tim and Christine Morgan, who presumably playtested it.
Matt's GURPS page: Variant game mechanics and rules simplifications.
MyGURPS: House rules, sample writeups and traits, GM aids, and reviews.
Richard's Cavalcade of Whimsy: GURPS variants and random monster abilities.
Self-Esteem and Repartee in GURPS Swashbucklers: Steffan O'Sullivan's adaption of an innovative concept from Lace & Steel, neatly ported to GURPS.
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.