The Trireme Battles blog offers a set of free wargames rules for trireme battles.
Miniatures Rules
Beer and Pretzels WWII Micro Armor
GHQ has a pdf of its Beer and Pretzels WWII Micro Armor, originally a commerial set, now a set of free wargames 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.