I Want
Fallout 3 (Xbox 360)
The freedom may scare people used to more linear shooters and the simplified customization might disappoint hardcore RPGers, but if ever a game was worth broadening your horizons for, this ...
Rock Band 2 (Xbox 360)
This refined entry replaces the original Rock Band as quite possibly the most fun-filled four person co-op game in creation. Just don't go looking for a huge evolutionary jump.
I'm playing
Call of Duty: World at War (Xbox 360)

Treyarch ditches the number subtitle and expectedly returns to WWII and the Pacific theatre of war. World at War has extremely brutal gunplay and amazing online multiplayer and co-op modes ...

Rock Band (Xbox 360)
This is the perfectly polished, logical extension of what Guitar Hero started and where the music genre needed to go.
I've Beaten
Mirror's Edge (Xbox 360)

A unique and enjoyable re-imagining of the First-Person Action Adventure genre, though a little rough in some small bits (also be wary if you suffer from motion sickness).