Major Nelson and Gear Box Software have just made a free Golden Key available for Borderlands 2 on the Xbox 360.
The SHiFT code will only last a few hours, so it would be a good idea to use it pretty snappily.
To redeem a code, select "Extras" on the main menu of Borderlands 2 and choose "SHiFT Codes" then follow the on-screen instructions.
SHiFT requires Xbox LIVE Gold and the account user must be at least 18 years of age.
For further details about what a Golden Key is and how/where to redeem is, hit the Gearbox forums
Read more at MajorNelson.com here.