Operation War Diary: An impressive project that hopes to make 1.5 million of pages of unit war diary page accessible to all.
Historic Newspapers: Teaching History Resources: A selection of three selections from important historical newspapers, available free to education facilities. The First World War option covers the Autumn Offensive of 1915, the death of Edith Cavell, the appointment of Sir Douglas Haig as commander of the B.E.F., the Dardanelles, Jutland, the death of Kitchener, Air Raids on London, the capture of Jerusalem and the Armistice. The side articles are just as interesting as the main stories, giving a feel for what was seen as important at the time. A super teaching resource.
World War One: Trenches on the Web, a very good website dedicated to the First World War.
Veterans Affairs Canada: First World War. The website of the Canadian Ministry for Veterans, it's first world war section has a nice selection of interviews with veterans and extracts from diaries.
The Great War: 1914-1918. A superb, well researched site with a fantastic section on the second battle of Ypres.
World War One in the News. A website that keeps track of news stories related to the First World War
Soldier’s Mail: Letters Home from a New England Soldier 1916-1919
The Long, Long Trail: the story of the British Army in the Great War. A very good site on the British Army, it's organization, battles, commanders and soldiers.
National Library of Portugal - article on the libraries that accompanied the Portuguese army on the Western Front
Supremacy 1914 - Online strategic multiplayer wargame set during First World War