Argad is a set of free wargames rules for skirmishes in the dark ages and for pirates in the 17th and 18th centuries.
Here’s a Horse and Musket DBA Variant — primarily for Napoleonic and Seven Years War, but since the basic principles of war in that era covered a huge number of conflicts, you could adjust as needed.
Island Mayhem is a set of rules for playing games in which 25mm – 28mm Pirates are searching for treasure.
Beyond the Saale is a set of campaign rules for the Seven Years War.
The Perfect Captain offers the second edition of John Bull / Patriots, their rules set for game of the American Revolution and War of 1812. These are the most visually striking rules sets available today — either commercially or free. And these are FREE!