Most Irregular is horse and musket rules extension to the Fast and Dirty skirmish rules, found on the same page.
Weight of Fire is a set of free wargames rules for horse and musket era battles (1740 - 1790). The rules are designed for 25mm - 30mm figures on a 1:25 scale.
Best Alles is a set of free wargames rules for 18th century games. To get the rules, you need to join the Yahoo Group.