OMOG Muskets is a set of free wargames rules for small unit actions in the horse and musket era, 1700 – 1849.
rules
Bladestorm Quick Start Rules
Bladestorm, the classic fantasy wargames rules from Iron Crown Enterprises in the 1990s is back in a second edition. You can get the quickstart rules at the link.
Wild West Exodus Free Core Game Rules
Wild West Exodus is a skirmish game that mashes the 19th century American West with steampunk and horror themes. You can get the free starter rules here.
Blaze Away WWII Rules
Blaze Away is a set of free wargames rules for company and battalion level actions in World War II. It’s designed for 10mm – 20mm figures.
Combat Action Science Fiction Rules
Combat Action is a set of free wargames rules for science fiction skirmishes. The author writes
Combat Action is a strategy game of small-scale conflicts. It can be used to simulate skirmishes with a dozen models per side or larger conflicts with multiple squads, vehicles and support weapons. The system presented in this book comprises two levels of detail: a system for small scale games where each model on the battlefield is a separate unit, and a second layer which adds rules that govern targeting, simulate chains of command and allow for larger engagements of forces. The point system will allow you to design any creature, vehicle or model you like and field any model you create on the battlefield. These rules are designed with the 25 to 30mm scale of miniatures in mind. Converting it to other scales of miniatures should not be a problem however, since there are few explicit range measures involved.