Black Cheats

BFG in first level:
Enter 5SQQ-5THA-ZFFV-7XEV as your profile name. Include the dashes. If entered correctly you should be able to enter a new profile name. You'll start the first level with the BFG.

Silver Weapons:
Beat the game on normal or hard difficulty. All guns for that difficulty and below will be silver and have unlimted ammunition.

Black Ops difficulty:
Beat the game on hard difficulty.

Extra Weapon:
Beat the game on Black Ops difficulty to unlock the M16A2 on any difficulty setting. It's an M16 with a grenade launcher attachment.