Battles fought in the Balkans during World War II

Military History Online - WWII: Several articles about the battles and campaigns of World War II. Also includes articles on other wars
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