Halo: Combat Evolved, Halo 2, and Halo 3 (XB/XB360)
The king of console multiplayer, and I absolutely cannot f*cking wait for Reach.
Left 4 Dead 1/2 (PC)
This game comes really close to toppling Gauntlet as my favorite co-op game. I remember that a lot of people complained (and even organized an official boycott) because they felt Valve didn't include enough content in the original game. However, I feel that I got my money's worth and then some. The expert campaigns are amazingly intense and versus is awesomely fun if you're playing with the right people.