Brutality is a miniatures agnostic skirmish wargame set in a science fantasy realm. You can get a free version at the link.
skirmish
Sixties Spy Skirmish Rules
Sixties Spy Skirmish Rules are a set of free wargames rules for James Bond style games.
Escaramouche Pike and Shot Era Skirmish Rules
Escaramouche is a set of free wargames rules for skirmishes in the Pike and Shot era. It’s designed to be played with perhaps six figures per side.
No Dachi Samurai Skirmish Rules
No Dachi is a set of free wargames rules for Samurai skirmishes.
Medieval Skirmish Rules
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.