Afterglow Skirmish Warfare In The Far Future is a set of free wargames rules for 28mm figures and is designed to be used with a variety of manufacturers.
Miniatures Rules
Naval Action
Naval Action, by Lou Coatney, is a set of free wargames rules for miniatures battles in the World War II Pacific Theatre. The page also provides some markers to use in place of model ships.
Battleforce Quick Start Battletech Rules
Battleforce is a set of free wargames rules for getting a quick start in Battletech.
Strongbow Feudal Warfare Rules
Strongbow is the Perfect Captain’s set of free wargames rules for feudal warfare from 1050-1300AD
Expanding Memoir ’44
Here’s an interesting bit on one gamers’ expansion of the Memoir ’44 system to a variety of conflicts.