USA 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
Lone Sentry: Very valuable resource, with scans from noumerous US photo albums, and transscripts of wartime US documents
Lt. Colonel Robert A. McDonald: Excellent biographical information about Lieutenant Colonel Robert A. McDonald, as well as thorough information about the rank insignia and awards of the US army during World War II
MHI Digital Library: Invaluable resource for the student of the US military. Thousands of pages of scanned documents, from the American Civil War to Desert Storm, including many about World War II. The only downside is that the documents take a long time to download, because of the magnitude, but it is definately worth the wait
Military Campaign Maps: Maps of military campaigns from throughout the US military history, including several from World War II
The Patriot Files: Large gathering of essays and diary entries from US soldiers serving in World War II, as well as all other US wars, giving an excellent insight into the mentality of soldiers on the battlefield throughout the ages
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
WW2WEB.COM: Interesting website, with some good articles about the US participation in World War II