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.
Miniatures Rules
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.
Clauswitz’ War: Theatre-Level Napoleonic Campaign Rules
Clauswitz’ War: Theatre-Level Napoleonic Campaign Rules is a set of free wargames rules for Napoleonic campaigns. The author writes:
These rules are designed to be used to represent theatre-level campaigning with armies of several corps per side during the Napoleonic period. Each turn is a day, and maps covering an area from 100 – 500 miles on a side with good detail (roads, rivers, towns) should be used. The intent of these rules is not to depict campaigning 100% accurately, but to provide a referee with basic guidelines for running a Kriegspiel-type game for the generation of tabletop engagements. All dice are ordinary 6-sided dice.