Red Army White Guards is a set of free wargames rules for playing battles set in the Russian Civil War. Its played on an offset grid and is apparently designed for 15mm figures; the rules call for three figures on a 30×30 mm base.
Rules: 20th Century
French Indochina War rules: Contre Les Viets
Contre Les Viets is a set of free wargames rules for fighting the French Indochina War.
Battlefleet 1900 Pre-Dreadnought Naval Warfare: 1890-1905
The War Times Journal has a set of free Pre-Dreadnought Naval Warfare rules, designed for 1/3000 scale miniatures.
Typhoon Modern Jet Combat
Typhoon is a set of free wargames rules for modern jet combat. From the author:
Typhoon is an introductory set of modern jet combat rules that does not require hex mats or any special flight stands for miniatures. It is designed as a quick playing, low complexity game for gamers who are interested in flying multiple aircraft with aircraft from the late 70s up to the Eurofighter and F-35.
Counterblast Adventure Battle Game
Counterblast is a set of free wargames rules for pulp-style adventures. The author writes:
Bombshell Miniatures presents the COUNTERBLAST™ Adventure Battle Game, a pulp-inspired science fiction tabletop skirmish battle game for 15mm or 28mm miniatures. COUNTERBLAST™ takes place in a dieselpunk setting of classic pulp sci-fi adventure with fast paced action and cinematic thrills, evocative of the serials that captivated the imagination of previous generations.