Rudi Geudens has a page with photos of World War One guns and tanks from the Belgian Royal Museum of the Army and Military History in Brussels. Photos such as these are always useful when painting your wargames miniatures.
In Clouds of Glory is a set of free wargames rules for World War One aerial combat.
14 – 18: Vistula, Verdun, Versailles is a set of free wargames rules for playing the first World War in 2mm scale.
This post on making WWI aircraft from clothespins is not really a set of instructions, but the photos show clearly enough what has to be done. Brilliant bit of model making.
I Wish I Was In France is a set of free wargames rules for WWI era games set in East Africa.