War Galley

War Galley is a set of free wargames rules for ancient naval warfare. It very cleverly uses matchboxes and matchsticks for play.

Link

Trireme Game Rules

The Trireme Battles blog offers a set of free wargames rules for trireme battles.

Link

Naumachiae Ancients Naval Rules

Naumachiae is a set of rules for ancients naval warfare for 1/12000 miniatures.

Link

Pz8 Wargames Rules

This 24 page pdf has Pz8’s relatively simple free wargames rules for a variety of periods, including

Colonial
WW1 Divisional
WW2
WW2 Divisional
1939 – 1941 Naval
WW2 Coastal
1935 – 1965 Aerial
1965 – 1075 Aerial
1950 – 1975
1975 – 2010
Science Fiction
Space Combat

Link

Shot and Sail Napoleonic Naval Rules

The prolific Jim Wallman offers a set of free wargames rules for Napoleonic Warfare called Shot and Sail. In addition to the pdf fo the rules, you’ll need the zipped graphics of ship cards.