GURPS Lite - A distilled, complete version of the GURPS Basic Set in 32 pages, available as a free download. Designed both for newcomers to GURPS and as a tool for experienced GMs to teach the game to others.
Roleplayer: The GURPS Magazine - An online archive of articles, adventures, and features from the sadly out-of-print SJ Games magazine/newsletter devoted entirely to GURPS. The complete (text) contents of the magazine's paper run is present.
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.
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.
Incanus' collection of scraps, tidbits, articles, and variants - dozens of GURPS items, including original work and articles culled from Usenet and sundry sources.