From the Junior General comes the Battle of Verdun Fast Play Rules. Designed for use with students, the rules are simple and fast. Paper figures also are available.
Dark World War is a set of free wargames rules — actually pay what you want — for playing WWI games that include demonic and mythological creatures.
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