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.
Raging Empires is a set of free wargames rules for WWI actions.
The authors write:
The Raging Empires rule system has been developed by WW1 enthusiasts to create a fast flowing game which captures the feel and flavour of this fascinating conflict.
This draft version of the rules contains all the game mechanics necessary to re-fight historical battles in all theatres and phases of the First World War, from the war of movement of 1914 to the major battles of 1916. This will be extended to cover 1917-18 in the full set of Raging Empires due for Autumn release.
One A4 Quick Reference Sheet is sufficient to contain all the information necessary to play. The main rules need only be consulted when players are learning the rules, and occasionally thereafter, to clarify situations as they arise.
Raging Empires is designed to be used for very large battles as well as relatively small encounters. The precision of the movement system and the speed and efficiency of combat makes the game flows quickly and even a closely fought battle of a moderate size can be concluded within 2 hours.
The game is designed to be played on modular hex terrain which can accommodate a range of different terrain features, e.g. open ground, trenches, strong points, hills and slopes, rivers, woods, impenetrable terrain etc.