Fokker is a one-page set of rules for playing miniature games with World War I aircraft.
Op14 is a set of free wargames rules for operational level game set in the World War I. The game is designed for figures on 15mm frontage bases, played on a grid.
Origins of World War I is a free print-and-play game by legendary wargames designer James Dunnigan. The game was re-implemented as Origins of World War II, an Avalon Hill game now long out of print. From the description:
A political/strategy game in which five players represent the five major nations in pre-World War I Europe. They must compete against each other to achieve each nation's 'national objectives'. While actual war is not allowed in the game, the play should show quite clearly how tensions developed to such a high pitch in 1914.
Lou Coatney's France 1914 is a free print-and-play, hex-and-counter boardgame of the German attack on France in 1914.
Trench Broom is a set of free wargames rules for game set in WWI. The rules are designed for 54mm figures.