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.
Matt Fritz has written a set of quick play rules called Day of the Dreadnaughts for recreating the Battle of Jutland. The set includes links to paper models, in case you want to try it, but don’t hvae the appropriate miniatures.