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.
Wrath of the Gods is a set of free wargames rules for skirmishes with a classical mythological flavor.
Realm is a set of free wargames rules for large scale fantasy battles.
The fourth edition of the venerable Chivalry and Sorcery RPG is available for free at RPG Now.
Wizards of the Coast offers the Dungeons and Dragons Elemental Evil Players guide as a free download.