Well I can see these people are gonna get lots of my money...

Twilight Creations. They make:
Zombies!!! - You vs the horde
Zombies!!! 2 Zombie Corps(e) - The horde on a military base
Z3 Mall Walkers - The horde invade the mall (BioZombie anyone?)
Z3.5 Not Dead Yet - More expansion cards
Z4 The End - The evil has been traced to a strange hut in the woods where you must collect pages of a mysterious book... Complete with Zombie Dogs.
Z5 School's Out Forever -  The Horde hits the local Uni.

And if that wasn't enough, you can "expand" the system even more by getting sucked back in time after Z4 (Sound familiar yet?) and fight the evil when it first appeared.
MidEvil - With Skeleton Hordes!
Midevil II: Castle Chaos - With MORE Skeleton Hordes!

So much potential for zombie slaying fun!