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.

Net books and guides from Olik's AD&D Pages.

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.

Min Max Boards: Gaming boards focused on D&D 3.5. Older posts can be found on Brilliant Gameologists.

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.