Skulltag (a doom port)
Super fun online multiplayer, and a very friendly comunity.
(But to play it you have to find one of the following "wads": Doom, Doom2, TNT, and more)
I love FPS, they are the best type of online games because you get to the action as soon as you start. Where in most RPGs you have to walk a hour to battle in a battle where your skills normaly dont matter...
I've refrained from on-line gaming, primarily because of social engineering and possibilities of downloading unknown scripting payloads. I know that this does not apply to all sites, but IMO ones I encountered I was not impressed. Games played are purchased ones from stores. I'd rather be safe than sorry.