In honor of winning the VGA's Game of the Year Award, Gearbox has put up SHiFT Codes that will be working for the next few days (specifics not given). so, without further ado, a-here we go! Happy hunting.
On the main title screen, click Extras then click "SHiFT Codes". on the next screen it'll give you a list of all the codes you've redeemed, and at the bottom there's a button to activate one. click that, then enter the code.