Counter-Strike is a given, even though I suck at it. FrontOffice Football (and the soon-to-be FOF: The College Years) is a great game.
Old School: Galaga (coin-op), Intellivision Baseball, PlayMaker Football, SimCity 2000 (think I wasted about 2 years of my college years on that one), and the greatest game of all time on any platform:
NHL '95 on Sega. (or was it '94?)