For about a year I have been playing this free game, Tremulous. The base of it is, you build a base with your team-mates and then attack.
For example, I would be the aliens, I would help build a better base (the stock bases are easy to defeat) andafter building a base, I would attack.
If anyone else playes this game, post here with your ingame name.
(Oh, yeah, it uses the Quake 3 engine but improves it fairly well...)