First World War dot Com is a good resource for doing research into the war to end all wars.
World War I
Jim Wallman presents Trench Assault, a battalion level game simulating a rush across no man’s land in 1916. Jim writes that it can be used with any scale figures, but that 1/300 or 6mm would be best because they are closest to the groudn scale.
Flying Circus WWI Air Rules
Flying Circus is a set of free wargames rules based on the old Avalon Hill Mustangs game.
Fast Play WWII Aerial Combat Rules
Matt Fritz offers a set of fast play miniatures rules for World War II Aerial Combat.
World War I Dioramas
Eye Candy time.
Mike Cooper has photos of several World War I dioramas that he has made. There’s Austrian Artillery, one of the Great War in Palestine, a trench mortar and a gas attack model.