Assault on Innsmouth is a set of free wargames rules for involving the Lovecraft Mythos. The players are government forces taking on the horrors in Innsmouth. The forces of the mythos are run by the system. The rules are a heavily modified version of the Two Hour Wargames system. There are also expansions at Board Game Geek.
The Jim Bowens Disturbia Yahoo Group has free rules for the Gotterdammerung WWII-Horror game.
Dead Walk Again is a set of free wargames rules for skirmish games in the zombie apocalypse.
Inquiry, Inc. is a set of free rules for playing games with Hasslefree’s not-Scooby figures.
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.