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 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.
Knights of the Sky is a set of free wargames rules for WWI Aircraft.
The Wargames and Railroads Blog has a nice article on how to paint a WWI US Marine. Very appropriate in this, the 100th anniversary of the start of the Great War.
Goodbyeee! is a set of free wargames rules for Great War skirmish actions.