Available until about 8PM Central time (One hour from this post) WinPC: 5BWB3-JWZRS-RT3JJ-BTJB3-RRJKH X360: 53KTT-KSJ5F-36XTF-KXTBB-XTCRC PS3: 53KTT-KSJ9B-69TWB-STWBB-XTC6B
There's a program you sign up for called SHIFT. You use these codes in the SHIFT section to get the free keys. If you own BL2, there are "Golden Keys" you can only get via promotions, etc. There's a golden chest in one of the cities that uses the Golden Keys. The chest is guaranteed (I believe) purple guns or higher.