Lethe:

If you're over 17:

Gears of War 2
Rock Band 2
Burnout Paradise (and all the old Burnout games available on Live or used for $10)
Fable 2
L4D (although I like it more on the PC)
Bioshock
Fallout 3

Finally, someone mentions Fallout 3. I would highly recommend it, it doesn't get old.