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.
Miniatures Rules
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.
21st Century Skirmish Rules
21st Century Skirmish is a set of free wargames rules for modern warfare.
Modernish Armor Rules
Felbrig Herriot offers his Modernish Armor Rules. The author writes:
These rules came about after deciding to buy some Ultra Modern 1/285th scale models of modern tanks. I couldn’t find any simple quick play rules so after referring to Gulf war anecdotes and of course Janes recognition guides I came up with these. They are simple to use and rely on the common D6.