Fearless is a set of free wargames rules that are:
… intended as a fast-paced science-fiction tabletop game including role-playing elements, with a universe full of unknown possibilities as the setting. It allows you to guide mercenaries from various backgrounds into breath-taking fights in the Arenas of Fearless. Additionally prepared scenarios and the possibility to customize your characters offer even more fun to play the game.
Cloudships of Mars is a set of free wargames rules based on the excellent Space: 1889 universe. The Cloudships game features ships propelled by steam and sail gliding high above the plains of Mars thanks to the magic of “Liftwood.”
Warlord Games has a free version of its Blood Red Skies WWII Aircraft Rules.
Contre Les Viets is a set of free wargames rules for fighting the French Indochina War.
Fusilier is a set of free wargames rules for the the flintlock era.
The author writes:
The Flintlock Era
Fusilier is a wargames system designed for the period 1700 to 1815.The system is designed to be used with any scale of figures. There is no formal figure scale, the unit sizes being dictated by the base sizes, which in turn are based on the frontages of the prototype units. An infantry battalion might be, for example, ten 25mm figures, twenty 15mm figures or fifty 5 mm figures. The ground scale is 1 mm = 1 metre