Zombies and Zork
Another multiplayer option for up to four people is Zombies mode. You can purchase items and weapons via points. You gain these points by killing enemy zombies and sealing passages. If playing with others, be sure to revive them before they are swarmed.
Shotguns can be great for blasting enemies down fast, but their long reloads can be a pain when you get surrounded. Starting with an assault rifle is always a great means of having balanced power and speed. You can always upgrade to those damaging sniper rifles and shotguns once you've earned enough points.
After completing campaign (Or using the cheat below), you will granted a new stage of Zombies where you can play as Kennedy, Castro and Nixon. The same Zombies rules apply, though you get a few different selections of weapons.
You can also access the computer terminal at the selection screen. Repeatedly press the iron sight and fire buttons and Mason will stand up from his chair. Move over to the computer on the left side and you'll be able to enter several codes: The code DOA will allow you to play Dead Ops Arcade, an over the head shooter. The code 3ARC UNLOCK will grant you access to Dead Ops Arcade and the Presidential Zombies mode right from the main menu. ZORK will allow you to play a full copy of the old PC game Zork. If you want to play through that game guides can be found here. Finally, 3ARC INTEL will allow you to read all Intel files.