American Civil War is a set of rules for 25mm figures in a 1:25 scale.
Border Town Burning is a free, unofficial supplement to the excellent, but abandoned, Mordheim, from Games Workshop. It’s got new rules, a new setting and lots of scenarios in a very professionally done format.
Classic Battletech is a free 24 page set of rules for playing the original game of giant robots on hex maps.
A Cast of Thousands is not so much a set of miniatures rules as a rules mechanic. It’s designed to allow you to play “cinematic games” where you set your own level of risk-and-reward.