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.