Sulphur is a set of free wargames rules for post-apocalyptic skirmishes. There’s an accompanying 28mm line of figures.
rules
Godlike Open Source Rules
Godlike – Open Source Rules is a conversion of the WWII Superhero RPG to the familiar d20 type system.
English Civil War Rules
MAWS offers a set of free wargames rules for the English Civil War. They’re meant for 25mm/28mm figures wehre one figure equals 10 men
Phalanx Ancients Rules
Phalanx is a set of free wargames rules for ancients. Designed for 6mm, 10mm, 15mm, 20mm, HO/OO and 25mm scales, the authors say it emphasizes speed of play while maintaining a good leval of accuracy.
Battle: War At Sea Modern Naval Rules
Battle War At Sea is a set of free wargames rules for modern naval warfare. It is designed for 1/6000 scale ships