Canvas Eagles is a set of free wargames rules for WWI Dogfights
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!
The Wire by Jim Wallman is not exactly a set of miniatures rules. Rather, it is a set of umpired rules designed to provide the participants with the feel of command during the Great War. Wallman writes:
This is the combat resolution system for a command post game about commanding brigades and divisions in WW1, particularly focussed on 1916. The rules are intended to be run in real time, and by umpires, who then write messages to the HQ players in the form they would have been historically, and subject to the delays and inaccuracies that would have existed at the time. The players never see the rules at all, and must work from a map (ideally in a smelly cellar by the light of a flickering oil lantern while umpires chuck buckets of earth at then).
Steve Burt offers a set of free wargames rules for WWI and WWII Naval combat.
Fokker is a one-page set of rules for playing miniature games with World War I aircraft.