The Great War Naval Rules is a very comprehensive set of rules for WWI Naval warfare.
Republished by Blog Post Promoter
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.
Here's a free ebook from Amazon that may interest some. Summary:
Francis Pegahmagabow has long been considered the most prolific killing machine of the First World War as he has been credited with over 378 deaths. This book chronicles his reputation as both a sniper and scout as the Canadian/British Army allowed him to roam freely throughout the "No Man's Land" region of France where he singlehandedly took down German troops. We also look at his later life as he suffered from "shell shock" (now known as post-traumatic stress disorder) and was deemed a "mental case" by the local Indian agents who oversaw his tribal band.
Spandau and Lewis is a set of free wargames rules for WWI aerial dogfighting.
Over The Top is a set of free wargames rules for brigade level games set in the First World War.