I'm going to suggest this game simply because I know no one else will:
Star Trek: Voyager: Elite Force
It is an older game (2000) so graphics leave room for improvement compared to today's games, but the gameplay and community support are amazing. I haven't played it in a year, but I know there are still servers running. The source code has been released, too, so the game is effectively free if you can find a download for it. Be sure to get the Expansion Pack as well!
If you really want graphics, you can try Elite Force II, but the game was never as popular so there aren't the plethora of community maps you would find for the first game.Edited by stargate125645 - 6/9/08 at 7:13am