Tribes 2 is back - http://www.tribesnext.com/
Best FPS game in my opinion, goes up to 255 players online. Now its free. No cd key needed. Runs off of linux material. Its for windows. This game is more of a team base games, Everyone is nothing with out being a team to do anything.
I never liked Tribes 2 to be honest. It always seems sluggish and desolate.
Starsiege Tribes on the other hand... best in the series