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 is a set of free wargames rules for WWI trench warfare.
Jutland: The Rules of the Game are a set of free wargames rules for large scale WWI Fleet Actions.
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.
Don offers an updated version of his air combat rules, which he now calls Aerial Craps.