Great Britain during World War II

European Center of Military History: Very extensive photo archive, with a lot of exellent photographs, primarily of Allied soldiers and materiel
War Letters: Scans and transscripts of many letters of US and UK servicemen from World War I and World War II. An excellent website for researchers
World War II Armed Forces - Orders of Battle and Organizations: Dr. Leo Niehorster's excellent website, with orders of battles for pretty much every country which fought in the war. His website also include errata pages for his books, and other interesting pieces of information