There are a slew of zombie flash games out there, and you can find many of them at
http://www.undeadgames.com/?action=play
"The Last Stand" is one I've heard mention of a lot lately, although frankly it doesn't strike me as all that great...once you get even a few of the weapons, you're pretty much unstoppable. Although I suppose that has its place, when all you want to do is shoot zombies. LOTS of zombies.
For those who demand a more professional zombie killing game, it has a lot of links to and reviews of commercial releases in the genre.