No Man’s Land is a solitaire card game that’s supposed to simulate being a sniper in the Great War.
Air War 1917 is a barebones — and free — version of Wessex Games’ AirWar: 1918 biplane combat rules.
If you’re out of game ideas, here’s a scenario generator for Spearhead: Great War.
First World War dot Com is a good resource for doing research into the war to end all wars.
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.