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.
rules
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.
Mars Or Die Rules
Mars Or Die is a set of free wargames rules for Victorian era games set on Mars.
War ’44 Rules
War ’44 is a set of free wargames rules designed for play with a seven year old. They would be good as an introductory game.