The battle for the Atlantic Ocean

Battle of the Atlantic: An outline of the Battle of the Atlantic, with information about the major events and combatants
Military History Online - WWII: Several articles about the battles and campaigns of World War II. Also includes articles on other wars
u47.org - the story of the "bull of Scapa Flow": Excellent website, in several languages, with very thorough information about the U-47 submarine, which attacked Scapa Flow in 1939
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
World War II day by day: Extremely detailed timeline of Germany during World War II, as well as many excellent articles, including a very large database of German vehicle markings
worldwar-2.net: Elaborate timeline of World War II, as well as many other interresting articles