Bravado is a set of free wargames rules for the horse and musket period. There are guidelines for both the American War of Independence and Napoleonic Wars for play. The rules employ an interesting mechanism where the players compete for initiative, deciding how much they will risk disaster in order to have another phase of movement or attack or time to reform their regiments.
Steve’s Horse and Musket Rules are designed to be a simple set of rules emphasizing the three arms (horse, foot, guns) and command.
For a limited time, Warlord Games is offering free digital copies of Black Powder, Hail Caesar and Pike & Shotte
6+ Rules are a set of rules for the horse and musket period in 6mm. It uses no dice
Cold Steel is a set of free wargames rules for the period 1690 to 1950 offered by Historic Enterprises. The first release offers the basic rules, plus specific addendums for the Wars of Napoleon and the War of 1812. The nice thing about the rules is that they offer a flexible scale system, with each figure representing between 5 and 60 men. This should allow you to fight a wide variety of battles with the same rules set.