Craig Grady offers a set of Samurai Skirmish Rules
Modiphius offers a free packet of factions for their Fallout Wasteland Warfare game.
Tin Army In The Civil War is a set of free wargames rules for the American Civil War by Ross Mcfarlane. The author writes:
This is a simple set of wargame rules aimed at Division or Corps sized battles. They assume that low level tactics are being handled by regimental officers and their Brigadiers and by the dice. The game may be played on grid with 1 unit per grid area otherwise all distances are roughly equal to the frontage of a deployed infantry unit or roughly 150 yards. Units within that distance and directly in front of, behind or beside each other are deemed to be adjacent. Turns represent a variable length of time and overlap the previous and following enemy turns. 12 to 15 turns each is a good medium length game with a simple scenario and a dozen or so units per side.
Gold, Swashbucklers and Pirates is a set of free rules for creating a small wargames campaign using gangs.
Iron Choirs is a skirmish game set in a fantasy post apocalyptic world. It’s a work in progress, but it is free right now at Wargame Vault. It could be of use.