This site includes the World War I Poetry Digital Archive. Everything in this site was: "Every item originates from, or relates to, someone's experience of the First World War, either abroad or at home. Contributions were received via a special website and also through a series of open days at libraries and museums throughout the country."
TEACHER OZ has done so much of the work for us all:
primary sources, images, songs, diaries, and much more.
(If the links are broken, do a browser search using the title of the page)
The First World War Poetry Digital Archive
Contains 6,500 items donated by the general public, including pictures, sound recordings, video clips, memoirs.
The Library of Congress (American Memory)Guide to WWI materials
There are lots of things here...rather hidden, so dig around for them!
National Archives / Docs Teach!
Video from World War I