World War I Document Archive http://www.lib.byu.edu/~rdh/wwi
An excellent place to begin and the fullest online collection of original sources on the subject.
Imperial War Museum http://www.iwm.org.uk
London’s Imperial War Museum is one of the two foremost museums dealing with the Great War (the other is its French counterpart in Peronne), and its website is especially strong on photographs.
Museum of the Great War http://www.historial.org
This site, for the French Historial de la Grande Guerre in Peronne (northeast of Paris), gives a good idea of the museum’s wide-ranging exhibitions and the scholarly research it promotes.
Trenches on the Web: An Internet History of the Great War http://www.worldwar1.com
A wide-ranging site with good posters, a reference library, and links to related websites on the war.
The Great War Society http://www.worldwar1.com/tgws
The Great War Society, which publishes a journal, promotes discussions, and organizes conferences, is one of the main organizations catering to individuals with an interest in World War I.