National Archives (UK)
Uk National Archives: First World War Sources for History
A comprehensive site filled with historically significant documents on the UK during World War I.
Brigham Young University
World War I Document Archive
This Brigham Young University website provides documents from World War I. Includes treaties, official papers, biographies, and images.
Library of Congress
Loc: Newspaper Pictorials: World War I Rotogravures
A collection of photos from newspapers during WWI as well as a timeline and essays on pertinent topics.