Square Hearts is a set of free wargames rules for colonial wargames in the 19th and early 20th centuries. It’s played on a square grid.
In 1975, Don Lowry released a set of rules called Grand Army. Designed for 9mm !? and 15mm figures, it allows players to recreate large scale battles for Napoleonics and the American Civil War. Grand Army now is available online.
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!
Stormed At With Shot and Shell is a set of free miniatures rules for playing games set in the Crimean War. They’re designed for 15mm figures.
Maxims and Mausers is a set of free wargames rules for European Wars from the 1870s. From the website:
Maxims and Mausers covers European Wars at the moment. Inclusion of Colonial Conflicts is planned. These are very simple rules. That may be played either as a board game on a hex map or as a miniatures game on large hexagons decorated as terrain.