A great deal of poetry was written about the Great War, much of it by soldiers in the trenches. Two period books of World War I poetry and poets are The Muse in Arms and For remembrance, available in a variety of formats at archive.org. There is also The First World War Digital Poetry Archive which mostly has things from the most well-known authors, but many of these are available as scans of the original documents. (The interface is a little iffy on the DPA; click on a person, then use the search for "any poem" to get a full listing of what's available)
The Great War Archive goes live today (November 11), the 90th anniversary of the Armistice. Launched by the University of Oxford in March 2008, the initiative invited members of the general public to submit digital photographs, audio, film, documents, and stories that originated from the Great War. Although the dealine for submissions is past, photos can still be added to the project's Flickr group.
On this day in 1915 the ocean liner Lusitania was sunk by a German U-Boat, which helped turn public sentiment in the US against Germany in The Great War. FirstWorldWar.com is your go to site for all things War to End All Wars related, from how it began to propaganda posters to maps to memoirs and diaries to the weapons and battles to audio and video and to the justly famous poetry of World War One. Also check out the feature articles and encyclopedia.
My Boy Jack. A heart wrenching story: "For Rudyard Kipling, the most famous author of the age, the carnage at Loos on the Western Front in September 1915 plunged him into inner darkness. His only son, John, for whom he had written his best-loved poem, If, had been killed in the action just six weeks after his 18th birthday." [more inside]