Zombies are the new Nazis in gaming. World War II is out, Zombie Apocolapse is in. They are the new thing that you can shoot and shoot and shoot without getting bored (that is, until something new comes along and replaces zombies).
Many current games now are about zombies or feature them in some way.
A zombie is either an actual undead (which is the case with Nazi Zombies), or a human who was infected with a virus of some sort and have undead/zombie like charactoristics (like Left 4 Dead).
Gears of War 2: Horde Mode
Call of Duty World at War: Nazi Zombies
Halo 3: Infection mode
Left 4 Dead
Resident Evil 5
Counter Strike: Zombie Mods