Links
Here are a few I have found useful:
Welcome to The League of WWI Aviation Historians
Shuttleworth Collection - The OFFICIAL Shuttleworth web site
Great War Aviation Photos
The Red Fighter Pilot, by Manfred von Richthofen
The Aerodrome
The World War I Document Archive
Public Record Office
A virtual museum of aviation artifacts
Cross and Cockade International
Over the Front
The Royal Air Force Museum
Commonwealth War Graves Commission
Smithsonian Institute
Cirrus UK Ltd. - Online