Cool mods that people actually play: Unreal Fortress, Weapons Factory UT, and JailBreak.
Cool mods that people don't play: F.E.B.A., U4E, ChaosUT, Airfight, Minions of Destruction, Dark Magic, NIUT... The list goes on.
Popular mods: Action Quake rips like Infiltration, Tactical Ops, etc.