NB: this announcement has been well publicised in the UK, so some of the linked websites are getting a lot of traffic, hence may open slowly for a day or two.
The UK National Archives announced today that it has completed the first stage of the digitisation of the war diaries kept by British Army units during the First World War. Every British Army unit had to keep a daily war diary containing reports on operations, intelligence summaries and other pertinent material. The hard copies of these are held at the NA in series WO 95, but it is now possible to see many of them online, albeit for a fee of £3.36 per diary.
The first stage sees the release of the diaries for the whole war of the seven infantry and three cavalry divisions sent to France and Flanders in 1914. As well as the diaries for the division, its component brigades, infantry battalions, cavalry regiments and supporting artillery, engineer, supply and medical units kept their own diaries. Diaries for units subsequently assigned to one of these divisions are included, not just those that comprised the divisions in 1914.
These diaries are the building blocks of most books written about the British Army in the First World War, going back to the Official Histories. Even books not based on archival research will have used other books that were based on these diaries. Note that these diaries are official military documents kept by unit adjutants. Some media reports seem to assume that they are the private diaries of individual soldiers.
A project called Operation War Diary has also been launched. Volunteers will analyse the pages that have been digitised. According to its website,
Data gathered through Operation War Diary will be used for three main purposes:
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to enrich The National Archives’ catalogue descriptions for the unit war diaries,
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to provide evidence about the experience of named individuals in IWM’s Lives of the First World War project
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to present academics with large amounts of accurate data to help them gain a better understanding of how the war was fought
Operation War Diary is a joint venture between the National Archive, the Imperial War Museum and Zooniverse, a company that has developed systems that enable volunteers to help with scientific and historical research from home.
See the BBC website and the Great War Forum for more details.
War and Security 
Ah, to be a professional historian and get into these resources.
I can’t speak for archives in the USA or other countries, but the UK National Archives are open to anybody over the age of 16. Many of its users are researching the history of their family.
http://www.nationalarchives.gov.uk/visit/default.htm