Wartime Journal offers 1916, a set of free wargames rules for battalion and regimental combat in World War I.
The Great War
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.
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.
WWI Bombed Out Copse
Roundwood’s World has some great photos of his WWI wooded terrain board.
No Man’s Land: A Solitary Game of WWI Sniping
No Man’s Land is a solitaire card game that’s supposed to simulate being a sniper in the Great War.