Matt Fritz has these free wargames rules for gaming the Battle of Midway. The rules are designed for 1/700 ship and airplanes models but Matt also provides paper counters and airplanes.
Pirates anyone? Pieces of Eight is a set of free wargames rules from Andy Watkins portraying Hollywood pirate skirmishes. They are a relatively simple set of rules — but that’s a good thing! Who wants complications when you’re playing pirates.
Majestic 12 games has these demo rules for their Grand Fleets miniature wargame. My group has played these and really enjoys them. Majestic 12 explains:
Grand Fleets is a comprehensive miniatures game of dreadnought-era naval warfare. In addition to rules for everything from ships to aeroplanes, Zeppelins to submarines, the rulebook includes:
Data cards for over 120 ships, plus guidelines for converting any ship of the period to game values; Eight scenarios encompassing conflicts from the Sino-Japanese War of 1894 through the opening salvoes of World War Two, including several variants; Fifteen pages of weapon data, covering all major gun and torpedo models in use through 1939, over 200 aeroplanes, and more than 20 airships; and Optional rules for flotillas, night-fighting, smokescreens, and more!
Red Banner is a set of free wargames rules for modern naval combat.
Corvus is a set of free wargames rules for fighting ancient sea battles.