Der Tag is a set of free wargames rules for WWI naval warfare in 1/6000 scale.
Rules: WWI
1916 World War I Rules
Wartime Journal offers 1916, a set of free wargames rules for battalion and regimental combat in World War I.
De Bellis Horribilis WWI Rules
De Bellis Horribilis is a set of free wargames rules for The Great War based on DBA.
Fokker Fodder
Fokker Fodder is a set of free wargames rules for World War One aircraft battles. The rules are designed for both single battles, and for campaigns.
To End All Wars – WWI Wargames Rules
To End All Wars is a set of free wargames rules for squad-based WWI miniatures games. The authors write:
To End All Wars is a miniature game that attempts to accurately recreate the First World War. Players control teams which have different roles in combat. They use a combination of teams to take positions on the battlefield and force their enemy from the trenches.