Flanders Field is a set of free wargames rules for the first World War. It lets players conduct regimental to divisional level actions.
The Great War
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.
Air War 1917
Air War 1917 is a barebones — and free — version of Wessex Games’ AirWar: 1918 biplane combat rules.