Steve Burt offers a set of free wargames rules for WWI and WWII Naval combat.
Miniatures Rules
The Last Human RPG
In this unusual free role playing game, you play a simple robot charged with defending the last human being. The Last Human is written by Nathan Russel as a part of the 24 Hour RPG Challenge.
1943 World War II Rules
From War Times Journal, 1943 is a set of free wargames rules for battalion level combat in World War II.
Red Actions! Interventionists
The Perfect Captain’s games are amazing combinations of form and function, with artwork that really matters to the game. They now have an expansion to their earlier Red Actions! Russian Civil War game that deals with the various nationalities that intervened in the strife. There’s also a new campaign.
Ancient Wargames Rules
Nick Salmon offers a set of free wargames rules for ancients warfare. The author writes:
These rules can be used from antiquity up until the late medieval period, they depart from the norm in that they have no rigid turn sequence. Instead players take turns to command their troops until a situation occurs which occupies the commander allowing the initiative to pass to the other player. Combat results are handled by progressively worse morale and no figures are removed from the table. As befitting my age the rules are a bewildering mixture of imperial and metric measurements. Any areas not covered should be resolved with common sense or a die roll.