Starsiege: Tribes
Tribes 2
Tribes: Vengeance
In the future (when released):
Rise (Crysis Mod)
Tribal Wars (or TWars, ET:Quake Wars mod)
Ascension (stand alone)
Tribes series: Fast paced FPS, with a real Z-Axis and 3D movement (no ground hugging). Jetpacks and Skis allow you build momentum across large maps in a futuristic setting. Three different armor classes, packs, and weapon loadouts allow you to choose your own style. Vehicles and base equipment like AI Turrets to protect your base. Very team-oriented.
The other three I listed are inspired by Tribes, and will be free.
I've tried other FPS (CS:S, CoD, Battlefield, etc.), and nothing has satisfied me as much as this series. Most challenging FPS I've ever come across...