Flanders Field is a set of free wargames rules for the first World War. It lets players conduct regimental to divisional level actions.
Miniatures Rules
One Page Napoleonic Rules
Land of the Lead has a set of one-page Napoleonics Rules. They’re designed for play with a “reinforced division” per side. Scroll down to the bottom of the page to find them.
Sulphur Post-Apocalyptic Battle
Sulphur is a set of free wargames rules for post-apocalyptic skirmishes. There’s an accompanying 28mm line of figures.
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