Zombie Escape is a Warhammer 40K modification for Imperial Guard versus Zombies
Dead Walk Again is a set of free wargames rules for skirmish games in the zombie apocalypse.
Fortress Figures offers “Zombie Plague”, a set of free wargames rules for horror scenarios. The free game goes with their excellent (and humorous) zombie plague figures. Check them out.
Invisible City Productions offers Zombie Rancher, “A post-zombie apocalypse game of entrepeneurisim for two or four players.”
Frag Zombies is a set of rules for combining Steve Jackson’s Frag game with Journeyman Press’ Zombies! board game. Of course, ownership of both sets is required, so I’m not sure how useful it will be.