Matt Fritz offers a set of fast play miniatures rules for World War II Aerial Combat.
Coal and The Kaiser is a set of free naval miniatures rules.
Author Leonard Heinz writes:
This is a game of low to moderate complexity portraying tactical naval combat in the period from 1904 through 1918. While it will not yet you play Jutland in a day, it will let you play some fairly large actions reasonably quickly. It uses a scale of 4 minutes to the game turn and 1000 yards to an inch on the playing surface. The rules focus on big ships and big guns: battleships (and particularly the Dreadnought-type battleships that began service in 1905), battlecruisers and armored cruisers.
Canvas Eagles is a set of free wargames rules for WWI Dogfights
Goodbyeee! is a set of skirmish rules for the Great War on the Eastern Front. Players command a force of 20 – 30 figures.
Entrenched is a set of free wargames for the early years of the First World War. It is designed for 1:76 scale figures.