David Heading offers a set of free wargames rules for Renaissance Naval Battles.
naval
War Galley
War Galley is a set of free wargames rules for ancient naval warfare. It very cleverly uses matchboxes and matchsticks for play.
Trireme Game Rules
The Trireme Battles blog offers a set of free wargames rules for trireme battles.
Naumachiae Ancients Naval Rules
Naumachiae is a set of rules for ancients naval warfare for 1/12000 miniatures.
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