It would help to indicate what specific game or games you're looking to join (3.0 D&D, 3.5 D&D, 4e D&D, etc.), and whether you're looking to play or GM/DM.
It also helps to check the message boards for whatever form of online RPGing you're looking into; for instance, if you're looking into play-by-post, you would check around EN World's Talking the Talk and Playing the Game forums, or the appropriate forum at WotC's website (I don't remember their PbP forum's name). If looking for a game over OpenRPG, it would be best to check the OpenRPG website's forum for Players & Game Masters. If looking for a game over MapTools, WebRPG, or something else, then check around their own forums.
If you're looking for a real-time game, rather than play-by-post, then it's also important to note what days/times you're available for game sessions.
For reference, though I don't know what you're actually looking for or when, I have an opening in my weekly Monday-night OpenRPG campaign of 3.5 D&D, For More Than Glory, and one or two openings in my weekly Sunday-night campaign of 3.5 D&D, Fall of the 14th Kingdom. I'm also sort of looking for another Shadowrun 3e player for my SR3 game on every other Saturday through OpenRPG. But that game's less consistent and runs earlier, around 4 pm Pacific, whereas the others run around 6:30 pm Pacific.