From Baccus Miniatures comes Gettysburg At The Gallop, a set of free wargames rules for the American Civil War in 6mm.
Command Combat Civil War: Bull Run is both a rules set and a scenario for playing the American Civil War.
I wonder why this hasn’t been done before. Eight Bells is a set of paper rules, combined with an iPhone app for playing Napoleonic naval games.
d12 Death Blow is a set of rules for man-to-man combat:
d12 Death Blow pits two combatants against each other in a contest for supremacy. The opponents might be martial artists, wizards trading bolds of eldrich energies, mentalists in a psychic contest or a pair of intergalactic wrestlers vying for the Cosmic Championship.
War In Space is a set of free wargames rules for star ship combat. What makes it different is that it uses standard playing cards for giving orders.