Tribes: Vengence (Very unique fps)
HEATHEN! 
Tribes: Vengeance was a fatal lobotomization of the franchise. It sacrificed much of the mod-friendliness, depth, and intelligent gameplay of its predecessors in exchange for brighter colors, a single-player campaign nobody cared about, and some sort of truck that looked suspiciously like a Warthog.
Tribes 2 was far better. If you haven't played it, (and you wouldn't have nominated Vengeance if you had) Sierra released the first two Tribes games as freeware back in 2004. Vivendi pulled the plug on the multiplayer authentication servers in 2008, but there's a fan-made patch to fix that.
/rant
Hey, I just noticed Tribes 2 was released in 2001! I'm going to nominate it.
I also notice that Mechwarrior 4: Mercenaries was released in 2002. It definitely deserves a nomination, especially when you factor in all of the epic fan-made multiplayer expansions. (Mektek is still working on their fourth mech pack, and was recently asked by the Mechwarrior rightsholders to release an official freeware compilation of the base game + all their mods)