d20 SRD - the rules of the game, presented in an excellent, readable format.
Roleplay on Slack 'n' Hash - Anyone still remember Agartha? Phil Smith revised this for 3.5, along with loads of other content. Check it out!
Creature Collection - A collection of monsters, converted to Third Edition.
D&D Adventures - "Over 300 adventures!"
FANCC - Fantasy Netbook Community Council.
Thibault Sarlat's Mystara site - Mystara resources, and a wonderful collection of maps usable in any setting.
Official Wizards of the Coast page - the obligitory link
Legal information is on my OGL page.