Loktar has a great site called Undead Games that mirrors a lot of Free/Open Source/Demos/Abandonware Zombie games. Visit his site here:
Some off the games he mirrors include:
Aftermath
All Hallows Eve
Autumn War
Bio-Nightmare
Blood
Blood 2
Boxhead the Rooms
BrainBread
Carmageddon 2
Crunch Time
Dawn of the Dead (remake) games
De-Animator
Dead EpidemicIn
Dead Rising
DeadHunt
Divine Intervention
DotD Mod
Dusk Of The Dead
Flesh Feast
Generic Zombie Shoota
Grand Theft Auto: Long Night
Grave 2
Horror Pack 3 for Operation Flashpoint
Horror Zombies from the Crypt
Horror Zombies from The Crypt
House of 1000 Identical Corpses
Infiltration
Iron Maiden: Ed Hunter
Killing Floor
Land of the Dead
Land of the Dead: Road to Fiddler’s Green
Last Night on EarthIn
Last Rites
Left 4 DeadIn
Loot ‘n Shoot
Monster Mash
Nearly DepartedIn
Nightmare Creatures
No More Room in Hell
Out of Hell
Over-Run
Project Necrosis
Quarantine
Resident Devil
Resident Evil 2
Resident Evil 3: Nemesis
Resident Evil Twilight
Resident Evil: Apocalypse
Rotting Onslaught
Stalker Horror Mod
Stickman Sam 2
Stubbs the Zombie
Survival Crisis Z
Swat 3 Rise of the Dead
Take No Prisoners
The Awakening
The Dead Awaken
The Devil Inside
The House of the Dead
The House of the Dead 2
The Hunted Chronicles
The Last Stand
The Typing of the Dead
They Hunger
Thing Thing Arena
Touch the Dead
Typing of the Dead Zombie Panic
Undeadgames Community Gallery
Urban Dead
We Are Legend
Whack the Dead
Y My 6th Movie
Zombi
Zombie 4
Zombie Erik
Zombie Grinder
Zombie Horde
Zombie Hordes
Zombie Hunter
Zombie Hunter II
Zombie Killer 2071
Zombie Killer 2072
Zombie Master
Zombie Nightmare
Zombie Panic
Zombie Patrol
Zombie Plague
Zombie Pox
Zombie Rally
Zombie Smashers X
Zombie Smashers X 2
Zombie Squirrel Attack
Zombie Stress
Zombie Survival
Zombie Swarm
Zombie Terror
Zombie Wars
Zombie Wars
Zombie Worms
Zombies of Nightmare
Zombies!!!
The whole list is here: