An Honorable Victory is a set of free wargames rules for battles set in Japan’s 15th and 16th century clan wars. The author writes:
An Honourable Victory! are an abstract (but not too abstract) set of rules designed for the table-top wargaming of the Clan Wars of Japan during the 15th and 16th centuries.
After one or two games you’ll have them down pat.
Dice For Honor is a set of free wargames rules for Samurai Skirmishes.
Jim Wallman offers Heroic Samurai, a set of free wargames rules for clan warfare in feudal Japan.
Sengoku Jidai is a set of quick play samurai skirmish rules. There’s also a Korean supplement.
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.