What's this?  Yes, it seems that developer Treyarch had a niftly little treat up their sleeves.  After you beat the single player campaign, you will unlock this little gem, which sends endless waves of Nazi Zombies after you.

It's a 4 player co-op level that only ends when you all die.  You and your teammates are stuck inside a building trying to fend off these Nazi Zombies.  You will earn money that you can use on weapons and items to build barricades to keep the zombies out.

I don't know about you guys, but this will more than likely be my most played game mode in Call of Duty: World at War.