The Great War Naval Rules is a very comprehensive set of rules for WWI Naval warfare.
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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.
Fokker Fodder is a set of free wargames rules for World War One aircraft battles. The rules are designed for both single battles, and for campaigns.
To End All Wars is a set of free wargames rules for squad-based WWI miniatures games. The authors write:
To End All Wars is a miniature game that attempts to accurately recreate the First World War. Players control teams which have different roles in combat. They use a combination of teams to take positions on the battlefield and force their enemy from the trenches.