The FidoNet AD&D Compendium Version 2.1: A collection of classes, monsters, spells, magic items and whatnot else for 2nd edition AD&D, created by the FidoNet community.
A whole bunch of random AD&D stuff, including worlds, character sketches and humor.
The Dungeon at druidic.net: A collection of interesting spells, feats, races and other D&D stuff.
IMarvinTPA's D&D Database: Indexed and searchable lists of spells, magic items and more.
Legacy Wizards D&D Boards: A great resource for players of D&D 3.5E, from Wizards of the Coast.
Wizards D&D Boards: A great resource for players of 4th edition D&D, from Wizards of the Coast.
Wizards d20 Boards: d20 System Boards hosted by Wizards of the Coast.
Macray's Keep: A complete gaming environment. Very well-designed site, with message groups, private mail and chat.
WebRPG: A huge online resource for gamers.
The RPG Gateway: An huge and incredibly useful collection of role-playing links.
Errant Dreams: A site much like this one, with useful tips on character and world-building.
Roleplaying Tips: A free online weekly magazine about roleplaying.