Sandarticus is a set of free wargames rules for backyard and sandbox games featuring Romans and their numerous adversaries.
Castle Cracker is a set of free wargames rules for backyard games in the ancient and medieval eras.
Roar of the Crowd is a set of free rules for playing science fiction, fantasy or historical gladiator games.
Matt Fritz offers a simple set of rules for recreating the 450 BC naval battle of Salamis between the Greeks and the Persians. The site even has links to paper models that you can use to play the game.
Matt Fritz offers a set of free miniatures rules called Across the Alps. The rules are for recreating the Battle of Trebbia from 218 BC, when Hannibal crossed the Alps to face the Roman Army at the river Trebbia. The rules are easy to play, being designed for Matt’s students. There also is a link there to paper figures.