Apart from the usual unlocks, Treyarch has hidden secret achievements, and unlockables in the game, which you can get by following the instructions below. In order to unlock these you need to get access to the computer terminal.
How to Get Access to Computer Terminal in Call of Duty: Black Ops
On the menu where you select Campaign or Multiplayer, use the Right Stick / Mouse to look down at your hands. Now click the Left Trigger and Right Trigger several times to get yourself up and walk around to the back right to the computer.
How to Unlock Zork Text Adventure Game in Call of Duty: Black Ops
Hit X at the computer and type in Zork to get the full Zork text adventure game.
How to Unlock Dead Ops Arcade Game in Call of Duty Black Ops
Exit out of the computer and then re-enter and type DOA to unlock Dead Ops Arcade Game.
“Five” Zombie mode mission, Dead Ops Arcade game, Zork game
Enter “3arc Unlock” to unlock the “Five” Zombie mode mission, Dead Ops Arcade game, Zork text-based adventure game, and all missions.
Enter “3arc Intel” to unlock all Intel but this disables achievements so no “Closer Analysis” for you.
Display all commands
Enter “help” as a code to display a list of system commands in the terminal and Pentagon user e-mail access.
Call of Duty: Black Ops Secret Achievements
Just ask me nicely (15)
Break free from the torture chair.
Eaten by a Grue (15)
Play Zork on the terminal.
Insert Coin (5)
Play Dead Ops.
See Me, Stab Me, Heal Me (5)
Fire a Pack-a-Punched Ballistic Knife at a downed ally to revive them from a distance.