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.
Corvus on Land is a set of free wargames rules to accompany the Richard Lee’s CORVUS (or any other Ancient Naval rules) when played in 1/1200 or smaller scales.
Aquanautica Imperialis is a set of rules for naval games in the world of Warhammer 40K.