Earth Expects is a set of free wargames rules for space combat. The authors write:
Earth Expects is a highly detailed set of tactical space combat rules which emphasise skill over firepower. It is well suited to playing battles from duels to squadron actions; because of the high level of detail large fleet battles can be slow to play and therefore optional rules are provided to speed play in these games.
The Perfect Captain’s Swamp Fox is a set of rules for American Revolution campaigns.
Hoplomachia is a set of free wargames rules for the Peloponnesian War.
2d6 Sword and Sorcery from Two Hour Wargames is available as a free pdf download. It is designed as a beer-and-pretzels style adventure game.
Titan Rampant is a free variant for the commercial Dragon Rampant rules. Ownership or access to Dragon Rampant is required for play.