Canvas Eagles is a set of free wargames rules for WWI Dogfights
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.
The Wire by Jim Wallman is not exactly a set of miniatures rules. Rather, it is a set of umpired rules designed to provide the participants with the feel of command during the Great War. Wallman writes:
This is the combat resolution system for a command post game about commanding brigades and divisions in WW1, particularly focussed on 1916. The rules are intended to be run in real time, and by umpires, who then write messages to the HQ players in the form they would have been historically, and subject to the delays and inaccuracies that would have existed at the time. The players never see the rules at all, and must work from a map (ideally in a smelly cellar by the light of a flickering oil lantern while umpires chuck buckets of earth at then).
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.