I think Fire and Fury is the finest set of miniatures rules ever written. It has elegantly simple mechanics, fun gameplay and set the standard for high-quality rules presentation.
Now the Fire and Fury people are at work on a version for America’s other “horse and musket” wars: The Revolution, War of 1812, and Mexican War. You can find the playtest versions of Fire and Fury: Americas Wars here.
Gold, Swashbucklers and Pirates is a set of free rules for creating a small wargames campaign using gangs.
Most Irregular is horse and musket rules extension to the Fast and Dirty skirmish rules, found on the same page.
Montcalm and Wolfe On Top of The Rock is a set of free wargames rules for gaming the Battle of the Plains of Abraham.
Blenheim 2004 is a set of free wargames rules for the War of the Spanish Succession. The rules are billed as lower-detail, designed for large battles.