Bug Hunt is a set of free wargames rules for Science Fiction Bug Hunts.
Rules: Science Fiction
Powerspike Science Fantasy Wargames Rules
Powerspike is a set of no-dice rules for science fantasy wargames.
Obydon 40K Rally Rules
Obydon 40K Rally is a set of free wargames rules for combat races using various Warhammer 40K vehicles.
JUGS: A Game of Big Stompy Robots
JUGS is a set of free wargames rules for science fiction combat with giant robots.
Giant robots. How can you go wrong?
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.