7th Air Force and the 78th Fighter Squadron on Iwo Jima
8th Air Force WWII Database
9th Air Force WWII Database
Army Air Forces in WWII
Aerofiles -- American aviation historical reference site.
Air Force Historical Research Agency (AFHRA), Maxwell AFB
Airworthy Warbirds and Vintage Aircraft Site
AMARC Aircraft Inventory List, Davis-Monthan AFB 
Aviation History On-Line Museum 
Dryden Research Aircraft Gallery (NASA)
Duxford Times -- Airfield news and information
Fleet Air Arm Archive -- Comprehensive Royal Navy history database.
Flight-History.com -- Archives of historic photographs, stories, aviation artwork and more
Flightline: Preserved U.S. Military Aircraft 
Globemaster "Tailcodes" Page -- Interactive database of military aircraft data.
Hangarline -- Canadian training aircraft histories.
Index of USAAF Heavy Bomber Groups of WWII 
Kilroy Was Here -- "Unknown Stories and Forgotten Places of WWII."
Military Aircraft Archive
Ministry of Defence (MoD), UK
On Camouflaged Wings -- "Issues and aircraft that have shaped military aviation."
Preserved US Military Aircraft List
Russian Aircraft Resource
US Air Force History Support Office, Washington, DC
WAE On-line Military History CD-ROM Catalog
WASP-WWII Website -- A chronicle of the Womens Airforce Service Pilots organization.
Naval Historical Center 
Aircraft Engine Historical Society

Aircraft Serial Number Databases:

Serial Numbers (USAAC/USAAF/USAF, 1922 to present) 
Serial Numbers (USN/USMC, 1917 to present)

Safety:

NTSB Accident Synopses
NTSB Accident Synopses (by Month)

Squadron and Groups

American AVG - Flying Tigers

Museums:

Museum of Flight
National Air & Space Museum (NASM)
National Museum of Naval Aviation
United States Air Force Museum