Goodbyeee! is a set of free wargames rules for Great War skirmish actions.
WWI
Flanders Field WWI Rules
Flanders Field is a set of free wargames rules for the first World War. It lets players conduct regimental to divisional level actions.
Trench Raider: WWI Waraming In Ten Minutes
Trench Raider is a set of free wargames rules for WWI trench warfare.
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.