Unreal Tournament, Unreal II, Quake III Team Arena, the soon-to-be-released Doom III (which looks like it will scare the hell out of you), and if you're into flight simulators and want to shoot somebody down, get Falcon 4.0 off of ebay. (It's not available in stores anymore but the online support community for this sim keeps it alive, vigorous, and up to date.)
CJ