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.
Iron Hands has an illustrated article on making trench systems for your 25mm – 28mm miniature wargames. The principal ingredients are concrete patch, rocks and fiberboard.
From the Junior General comes the Battle of Verdun Fast Play Rules. Designed for use with students, the rules are simple and fast. Paper figures also are available.
Dark World War is a set of free wargames rules — actually pay what you want — for playing WWI games that include demonic and mythological creatures.