Rainbow Six - from what I played, looks like Counter-Strike all over again.
Battlefield 1942 is a lot more epic and you can have huge battles in that game. Lots of fun at first, then just gets boring. HUGE Community!
MoHAA - excellent online, depending on which mode you play anyway. Getting old now, so the community is getting smaller. Again, it plays a lot like Counter-Strike. Lots of cheating online too
Moved to the Multiplayer Forum