No Dachi is a set of free wargames rules for Samurai skirmishes.
Richard Bradley’s Medieval Skirmish rules are offered for free on the Tyneside Wargames Club site. There actually are quite a few free wargames rules there. All look quite playable and fun.
Richard Bradley of the Tyneside Wargames Club offers a set of free wargames rules for skirmishes in the English Civil War.
Here’s a set of rules for converting the large scale Warhammer Fantasy battles game to a skirmish version.