Gearbox Software will be giving out SHiFT Codes to Borderlands 2 players which can unlock Halloween themed character skins. A few days ago they gave away free Golden Keys through SHiFT Codes, Golden Keys give you a chance to get rare loots on the Golden Chest at Sanctuary. They will start giving out codes on October 31, 2012 at around 10:00AM CT (11:00PM here in the Philippines), you can get the code through their Facebook page and/or Twitter account.
Take note that the codes will be active until November 4 at around 11:55PM CT (Nov. 5 1:55PM in the Philippines). For more information on how to get the SHiFT codes, you can go to this link for the whole details.