British Pathe's collection of World War I films
The First World War Poetry Digital Archive (Oxford University)
World War I (British Library)
World War I At Home (BBC Four)
Lives of the First World War
Trench Maps and Aerial Photographs (McMaster University)
Mission Centennaire 14-18
See also NARA's Centennial site, Library and Archives Canada's site, and the Library of Congress's World War I web guide.