Red Orchestra's pretty good if you like the high-realism type of FPS. I don't play it too terribly often, but when I'm in the mood for some relatively authentic WWII action, it's the game I go to. Nothing like taking out a Russian tank from 1000 yards away with your Tiger after estimating range, adjusting the sight appropriately, and letting it fly. I love destroying the little russian "clown car" APCs with a tank while they're on the run.
RO also tends to have a much lower quantity of immature players, as younger geeks aren't too excited by the high level of realism. The game's at it's best when you're with a good, cooperative team that's communicating well with VOIP. If you're in a tank as a gunner or driver, and you know what you're doing and so does the other guy, and you're communicating well, you can really wreak some havoc. Of course, there's always the occasional smacktard who jumps in your tank you'd been in alone and drives off right when you were about to nail that perfect long-range shot.