Flanders Field is a set of free wargames rules for the first World War. It lets players conduct regimental to divisional level actions.
WWI
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.
WWI Bombed Out Copse
Roundwood’s World has some great photos of his WWI wooded terrain board.