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.
Giants in the Playground: D&D boards run by the creator of Order of the Stick.
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.
Sly Flourish: A useful collection of articles on building a better DM.
Homebrewery: Create RPG material in a format that looks like books from WotC
Obsidian Portal: I can't believe it took me so long to find this site. Join a hundred thousand gamers in a community which includes hosted campaigns, player and game finders, and more, for all RPG systems.
Giants in the Playground forums, mainly D&D.
Min Max Boards, for D&D and Pathfinder.
And you can find a wide variety of PBeMs here: http://www.pbm.com/~lindahl/pbm_list/PBM List
Find Gamers is a good place to find role-players and wargamers in your area or online.