Tom’s Ancient Warfare rules are a set of free wargames rules designed to be played with — of all things — Risk pieces. The author notes that the bases may need to be larger if you use historical miniatures.
Miniatures Rules
Aquazone Game of Deep Sea Exploration and Combat
Aqua Zone is a set of free wargames rules for playing miniatures games that feature underwater combat with submarines, DSV’s and marine animals.
Faction Wars Science Fiction Skirmish
Fire and Forget Games has released a downloadable PDF version of the rules for Faction Wars, the sci-fi skirmish game. The free 40-page PDF contains the basic rules of the game, sample squads for two factions, and a couple of scenarios to get you started.
Minifigz Lego Combat Rules
Minifigz is a set of rules for combat with Lego figurines.
Elfball Fantasy Football Rules
From Impact Games, Elfball is a set of fantasy “football” rules. It’s played on an oval field, so it’s not like “American Rules.”