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
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.
Fleet of Battle is a set of free wargames rules for playing out battles between fleets of ancient galleys. Think Ben Hur. There also are several pages of free print-and-play galleys for use in lieu of lead miniatures. The rules require a knowledge of the commercial Field of Battle wargames rules.
Ironclad Draughts is a set of one-page rules for playing out battles between ironclads.
Periplus is a free two player wargame that abstracts a battle between two fleets of ancient greek warships.