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.
Clay Kickers is a set of free wargames rules for tunnel warfare in World War I. The authors write:
Ypres, 1917. A great battle rages overhead, the like of which the world has never seen before. An unceasing exchange of industrial Firepower, grinding men into ash and dust. Beneath the earth, a different kind of struggle is waged. Men cram themselves into lightless burrows, undermining the great earthworks above with great quantities of explosives. This is the story of that struggle. Clay-Kickers is an objective based skirmish wargame for two players. Each player controls a squad of engineers, demolition experts and battle-hardened close-quarter blighters. With their motley crew of tunnel rats, the players will create their own tunnels under the battlefields of the Western Front, avoiding underground hazards and engaging the enemy in brutal close-range brawls.
Der Tag is a set of free wargames rules for WWI naval warfare in 1/6000 scale.