The First World War Poetry Digital Archive. From the University of Oxford. Don’t be fooled. This site has a lot more than poetry. It contains scans of actual letters, diaries, maps, postcards, and more. The “browse by keyword” feature is helpful.
Library of Congress. The digital collections of the Library of Congress contain a wide variety of material related to World War I, including photographs, documents, newspapers, films, sheet music, and sound recordings.
British National Archives Official UK government records of the First World War, including a vast collection of letters, diaries, maps and photographs.
University of Wisconsin Digital Collections. “This collection includes published materials as well as archival documents. The items were digitized from a variety of formats including books, manuscripts, sound recordings, photographs, maps, and other resources.” Tip: search “only full-text” and use the browse link to find WWI subcollections.