GHQ has a free pdf of its Beer and Pretzels Modern Micro Armor game, formerly a commercial effort, now a set of free wargames rules.
GHQ has a pdf of its Beer and Pretzels WWII Micro Armor, originally a commerial set, now a set of free wargames rules.
Age of Fantasy: Regiments is a set of free wargames rules for simple fantasy Regimental actions. The rules fit on one page, and there are “army books” for various races.
War ’44 is a set of free wargames rules designed for play with a seven year old. They would be good as an introductory game.
Simplus Medieval Miniaturis Rules (SMR) is set of free wargames rules for battles in the medieval era.