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.
I Wish I Was In France is a set of free wargames rules for WWI era games set in East Africa.
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.
On The Firing Line With The Germans is a feature-length documentary shot in 1915 by two American filmmakers to show the Great War from the German perspective.
Thanks to reader Peter Schweighofer for the tip pointing me to the film. You should also visit his site.
Update: You may find that the YouTube copy of the film streams better than the one from the dot gov site. It does for me.