GHQ has a pdf of its Beer and Pretzels WWII Micro Armor, originally a commerial set, now a set of free wargames rules.
Miniatures Rules
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.
Reconstructed Sweet Home Rules For Ancient Battles
Blunders on the Danube Blog has a set of free wargames rules for ancients battles based on Charles Sweet’s classic set.
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.