By Jeff Peach & Les Booth offer a set of free wargames rules for ACW River and Coastal Warfare
Here’s a set of free wargames rules for the Spanish American War using multiple dice sizes and designed for 28mm figures.
Atteint! is a set of free games rules for medieval jousting.
Kings of War is a great free fantasy rules set from Mantic Games. Here’s a variant for the Viking Age
Space Fleet 2014 is a set of free wargames rules for fleet actions. It can be played on a hex grid or open tabletop, and there are rules for ship design, campaigns, sensors, and more.