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.
First World War
First World war.com
Site does a good job of giving an overview of most of the key aspects of World War I: the causes of the war, a timeline, weapons of war, etc. As a special feature you can listen to vintage audio of speeches and music of the WWI era.