There is also which might work well with some gridded World Works sets. And includes 100 plastic zombies to shamble at the brave(?) heroes.

The Auld Grump

*EDIT* By the way, this is a fun game!

*EDIT 2* Fixed link

Yeah, Zombies is a fun little game. The zombies themselves are rather small; probably 20mm or so. They may look ok with Urban Mayhem if you had other figs about the same size. A zombie infested Mars base sounds like fun, too.

Monkey, I take it you're looking to create your own game?

Something to think about on the squad vs. individual mini design is how the zombies organize themselves. Is it a hive mind of sorts? Then a squad based system would fit very easily. Are the zombies still capable of some kind of independent thought? Then going with individual minis would fit.