Shooter fan? Gears of War - This is a great game, the images can just be amazing at times!
Horror/Zombie fan? - In my opinion Dead Rising is probably one of the better games (until they bring resident evil 4 to the 360 - wishful thinking) I ever played in the genre.
I also have to agree about the Live Arcade - some of those games can get addicting and some of those games are REALLY original like cloning clyde and others. And nothing beats worms blowing each other up!