Tanzanica is a set of free wargames rules for Darkest Africa adventures by Brett Abbott. The rules actually are a variant of Games Workshop’s Mordheim miniatures rules, which are themselves free at the Games Workshop living rules archive. Mordheim is a neat set of rules for the kind of game where each player controls a small group of characters, with one uber-character and several flunkies.
Slayer of Vampires is a set of free wargames rules for skirmishes set in the “Buffy” universe.
Strange Aeons is a skirmish wargame of Lovecraftian horror. You can get a Strange Aeons demo kit here, and the quick reference sheet for the second edition Strange Aeons here.
Dead Walk Again is a set of free wargames rules for skirmish games in the zombie apocalypse.
Scavenge Skirmish Survive is a set of free wargames rules for a tabletop game set in a post-apocalyptic volcanic winter. Link goes directly to a pdf