War Galley is a set of free wargames rules for ancient naval warfare. It very cleverly uses matchboxes and matchsticks for play.
The Trireme Battles blog offers a set of free wargames rules for trireme battles.
Naumachiae is a set of rules for ancients naval warfare for 1/12000 miniatures.
This 24 page pdf has Pz8’s relatively simple free wargames rules for a variety of periods, including
1939 – 1941 Naval
1935 – 1965 Aerial
1965 – 1075 Aerial
1950 – 1975
1975 – 2010