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.
Ok. This is neat. It’s a Java window that shows how fast one infected individual can destroy a population.
Zombie Infection Simulation v2.3
Living Dead is a set of free roleplaying rules for the zombie apocalypse.
Unheroic Samurai is a set of free wargames rules for combat with classical Samurai versus undead horrors.
Table Top Titans have released Twilight, a set of free miniature wargaming rules for skirmishes in a zombie plagued world.