Doom 3 was really good, I liked it. Halo is great for multiplater (halo 2 is X-box only) Half-life 2 is awesome, Chronicles of Riddick: escape from Butcher Bay is an AWESOME game (hint, it's only $20), Farcry was also really good (refreshing after playing Doom3), and so far Starwars: Republic Commando is really cool. Being able to command a squad of commandos is pretty neat.
All the above mentioned games are first person shooter.
If you like real-time stratgey games, War Hammer: Dawn of War is excellent, as is War Craft III.
BTW, you'll need a fairly beefy computer to play any of the FPS games listed above, don't cheap out on the vid card.