Cuirassiers 1700-1870 is a set of free wargame rules for 18th and 19th century warfare. It is ideally for 6mm figures.
Into The Breach is a set of free wargames rules for skirmishes in the horse and musket era.
Here’s yet another set of free wargames rules based on DBA. These are for 18th Century European and Colonial Warfare in 6mm.
Weight of Fire is a set of free wargames rules for horse and musket era battles (1740 – 1790). The rules are designed for 25mm – 30mm figures on a 1:25 scale.
Natholeon’s Empires offers a set of free wargames rules for the Franco Prussian War.