If you're looking for a good overview of World War II records, then militaryindexes.com is for you. In one lovely concise page, they've listed free and commercial resources for World War II records.
One of the links is to the enlistment records in the Electronic Army Serial Number Merged File, ca. 1938 - 1946, a searchable database of 8,706,394 records at the National Archives. Other NARA databases include the Records of World War II Prisoners of War, created, 1942 - 1947, documenting the period 12/7/1941 - 11/19/1946; World War II Prisoners of the Japanese Data Files, created, 4/2005 - 10/2007, documenting the period ca. 1941 - ca. 1945; Records About Japanese Americans Relocated During World War II, created, 1988 - 1989, documenting the period 1942 - 1946; and Records of Duty Locations for Naval Intelligence Personnel, created, 1942 - 1945, documenting the period 1942 - 1945.
Other links on the militaryindexes.com page for WWII include the Ancestry and Fold3 commercial sites and a list of WWII resources by state and by country.
Another useful link explains how to order records from the National Personnel Records Center in St. Louis.
If you're just starting your WWII research or you need some fresh ideas, the Online World War II Indexes & Records site can help.