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.
14 – 18: Vistula, Verdun, Versailles is a set of free wargames rules for playing the first World War in 2mm scale.
I Wish I Was In France is a set of free wargames rules for WWI era games set in East Africa.
Coal and The Kaiser is a set of “low to moderate complexity” rules for surface naval combat in the 1904 – 1918 era. The author writes:
This is a game of low to moderate complexity portraying tactical naval combat in the period from 1904 through 1918. While it will not yet you play Jutland in a day, it will let you play some fairly large actions reasonably quickly. It uses a scale of 4 minutes to the game turn and 1000 yards to an inch on the playing surface. The rules focus on big ships and big guns: battleships (and particularly the Dreadnought-type battleships that began service in 1905), battlecruisers and armored cruisers. Rules for smaller ships are meant to facilitate fast play and authentic tactics. I consider the rules to be at the beta test stage; I’ve tested them myself over a period of three years, and have started to play test them with others face to face.
The Great War Naval Rules is a very comprehensive set of rules for WWI Naval warfare.