Blaze of Glory is a set of free wargames rules for Wild West skirmishes.
Rules: 19th Century
Heart of Oak, Skin of Iron
Heart of Oak, Skin of Iron is a set of free wargame rules for naval combat in the years 1852 – 1870.
Splendid Little War
Splendid Little War is a set of free wargames rules for the Spanish-American War.
Old West Miniature Wargaming
Old West is a set of free wargames rules for wild west skirmishes.
The main rules file on the Internet Archive capture of the page is in Postscript format, so I converted it to a standard PDF. You can download it below.
Cold Steel Miniatures Rules for 1690 to 1850
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.