I had reason recently to do some research on an ancestor of my husband who died at St. Mihiel during WWI. Along the way I collected these links to online material regarding The Great War, and thought I would share!
* Great War Archive from Oxford University includes original documents, images and recollections submitted by individuals. You can also check out the archive's Flickr group for photoes, posters and more.
* BYU's World War I Document Archive includes personal memoirs and diaries.
* Site linking to various personal diaries posted online. The site is hideous, but the links work, so no fuss!
* Great War Photo Archive has over 3,000 photos covering different subjects, including weapons, high ranking individuals, and troop photos.
* Color Photos from WWI, mainly from the Western Front
* Aftermath, which focuses on the legacy and ramifications of WWI both on the personal and international level.
Not personal diaries, but unit diaries which detail actions of individual units during the war
* Canadian War Diaries from the Canada Library and Archives
* Surrey Regiment Diaries