14 – 18: Vistula, Verdun, Versailles is a set of free wargames rules for playing the first World War in 2mm scale.
I Wish I Was In France is a set of free wargames rules for WWI era games set in East Africa.
Coal and The Kaiser is a set of “low to moderate complexity” rules for surface naval combat in the 1904 – 1918 era. The author 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. Rules for smaller ships are meant to facilitate fast play and authentic tactics. I consider the rules to be at the beta test stage; I’ve tested them myself over a period of three years, and have started to play test them with others face to face.
The Great War Naval Rules is a very comprehensive set of rules for WWI Naval warfare.
Here’s a tutorial on scratch building a 15mm mortar.