Off the top of my mind,
For car games, Midnight Club LA. is the sickest game... Ever... By rockstar, who also made GTA shizzle.
I enjoyed Gears of war2 and 3, havent really played the first one,
halo 3 is sick as fuck, multiplayer matchmaking is perhaps the coolest online FPS i know of.
Jazz Jackrabbit 2 for PC (download it noob) is a sick oldschool rabbit platform shooter game.
Jibbin'. Just make sure to download some grabpacks and steeze stuff.
Ninja gaiden 1, and 2. Sick games. Really.
All the skate games are cool. I particularily enjoy skate 2, for the fun track. If you just do a regular 180, 540, 900 etc, as long as u land switch, you can do a cork on your next jump, just when you land your 180/540 etc, prepare for the next jump asap when you land, so you get into your next jump switch..
I might post some more later if i remember some more gems