Unreal Tournament is a legend in PC first person shooter land. It's up there with Doom, Quake or Counter Strike amongst the most successful FPS franchises. What makes Unreal unique is the extremely fun multi-player mode, with it's fast paced action and futuristic weapons of mass destruction.
It's games like Unreal Tournament that make people become gamers. If you call yourself one, stop reading and go buy UT3 now.