Rabbidgerbal’s Super Simple 1PG Sci-fi Skermish Rules is a set of free wargames rules meant as a 40K replacement.
Yes, that’s really how the author spelled “Skirmish”
Cold Steel Miniatures Rules for 1690 to 1850
Cold Steel is a set of free wargames rules for the period 1690 to 1950 offered by Historic Enterprises. The first release offers the basic rules, plus specific addendums for the Wars of Napoleon and the War of 1812. The nice thing about the rules is that they offer a flexible scale system, with each figure representing between 5 and 60 men. This should allow you to fight a wide variety of battles with the same rules set.
Ambush Guerrilla Wargaming Rules
Ambush is a set of free wargames rules for guerrilla warfare 1960 – 2000.
14 – 18: Vistula, Verdun, Versailles
14 – 18: Vistula, Verdun, Versailles is a set of free wargames rules for playing the first World War in 2mm scale.