Panzer Troops is a set of free wargames rules for WWII battles. The rules are intended as an introduction to historical wargaming.
Dog Soldiers is a set of free wargames rules from the old Gauntlet for the plains wars.
Platoon Action is a set of skirmish rules for WWII. The author writes:
Platoon Action is a skirmish-level game of World War II combat. Figure scale is one man to one man; ground scale is not exact, but about 1" to 5-10 yards. Typically a player might control up to an infantry platoon (three squads of about ten men each; perhaps an HQ section); optionally supported by a few heavy weapons or vehicles. Although there are vehicle rules, it should be noted that these are primarily rules intended for infantry combat.
Fire In The Fern is a set of free wargames rules for games set in colonial New Zealand.
Simplis Medieval Miniaturus is a set of free wargames rules for miniatures battles set in medieval times.