Trench Raider is a set of free wargames rules for WWI trench warfare.
world war One
Jutland: The Rules of the Game
Jutland: The Rules of the Game are a set of free wargames rules for large scale WWI Fleet Actions.
Mimi and Toutou Go Forth WWI Naval Rules
Mimi and Toutou Go Forth is a set of simple naval rules for WWI naval battles on Lake Tanganyika. Obscure, to be sure, but intriguing.
WW1 Air Combat Rules
Don offers an updated version of his air combat rules, which he now calls Aerial Craps.
WWI Fantasy Rules
Orks and Dwarves in a WWI setting? Sure. Here’s a set of fantasy WWI Rules.