Wartime Journal offers 1916, a set of free wargames rules for battalion and regimental combat in World War I.
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.
Flying Circus is a set of free wargames rules based on the old Avalon Hill Mustangs game.