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.
BBC
Bbc: History: British History Timeline: World War I
Enter "1914" in the Search box of this year-by-year timeline of British history to trace the progress of World War I, and see other significant events that happened during the war years. Archived.
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.