Which older shooter had a separate zombie shoot (as a bonus) once you finished the real game? I forget, but I think it was a popular title. You just stood in a room and blasted the undead before they could get in?
Call of Duty: World at War or Black Ops. Though in Black Ops it was available right off the bat. In World at War various ***** Zombie maps unlocked as you played through the campaign. And given CoD:WaW came out in 2008 I'd figure it's the one you're thinking of.
Call of Duty: World at War. The USA in the Pacific , and Russia in Stalingrad. Here's the second mission: http://www.youtube.com/watch?v=A4N4c6s3o_s By the time it came out, people were getting sick of WWII games, but still, it was very good. Also, probably the nicest graphics of all the WWII games. Once you finished the single player campaign, just for fun, you unlocked a bonus where you defended a building being attacked by ***** zombies trying to get in. No real strategy, just fun violence. Off the wall thing for such a game, but enough to make you smile. Here's a zombie attack: http://www.youtube.com/watch?v=FsKdbWNi3cI It's pretty cheap these days, and well worth picking up.