Kensi is a set of free wargames rules for samurai warfare ranging from skirmishes to large battles. The rules include a fantasy element, and the world is a not-quite-historical Japan.
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Valley Of The Apes Miniatures Adventure Game
Valley of the Apes is designed as an adventure game for kids. Players create their own jungle terrain, scatter the encounter tiles (with hazards or treasures) across the valley, and send bands of toy soldier figures to seek fame and riches.
High Crusade Science Fiction Rules
High Crusade are a set of free wargames rules for 15mm / 25mm science fiction games.
Wrath of the Gods Fantasy Skirmish Game
Wrath of the Gods is a set of free wargames rules for skirmishes with a classical mythological flavor.
The Sword In The Philippines Rules
Larry Brom’s The Sword and the Flame may be as close as anyone has come to a “universal” wargames set. Here’s a variant of the game for the Moro Rebellion and the US Involvement in the Philippines: The Sword In the Philippines