Team Fortress is a revolutionary team and class based multiplayer first person shooter mod for Quake.
What else is there to say. It was 1997 and I met a buddy in highschool who introduced me to this game. The rest is history. Never before had I encountered a first person shooter game with class based team play. You were always fearful of crossing the bridge in 2fort5 because of elite snipers.
As a spy you actually disguised as an enemy and could feign death. Enemies would hide in the shadows and wait for ambush. Grenades galore where available for each class. This is one of the best mods ever created for any platform.
One can only hope it is resurrected in the future.