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.
Slayer of Vampyres is a set of free wargames rules for recreating the battles in the classic TV series Buffy The Vampire Slayer.
Unheroic Samurai is a set of free wargames rules for combat with classical Samurai versus undead horrors.
RAFM offers a set of free wargames rules for skirmishes: Universal Soldier Extreme / Modern Day Heroes
Universal Soldier Extreme is a set of rules for skirmish level miniature gaming using Rafm Miniatures. The focus of the main rules is on the modern era and optional rules are provided to suit any genre ranging from horror, to pulp fiction, to historical combat. The rules focus on the Modern Day Hero’s line as well as the Cthulthu mythos creatures. The inspiration for these figures comes from the traditional H.P. Lovecraft Horror genre as well as modern movies and cinema. These rules are a skirmish level set of rules. This means that each player would command between five and ten miniatures at a time. Although flexible enough to handle many more miniatures it is up to the players to decide how big a game they desire to play. We have found that a game of this level with two players can be finished in one and half to two hours playing time.
Set in the pulp 1920s, Strange Aeons is a skirmish game that sets bands of adventurers against Lovecraftian horrors. The game is commercial, but they have a free test version.