Internet is dynamic and alive, so this list will always be under construction. I do not intend to list every website on the globe that touches on classic aviation. It's merely my personal selection of sites that I find very useful or of exceptional quality, or that I choose to include for any other reason.
Aviation History and Photography - Ruud Leeuw's excellent site about all aspects of classic aviation.
All About Guppys - The name says it all. If there's anything you want to know about any Guppy variant, check here!
Constellation Survivor Website - Ralph Pettersen's magnificent website on surviving Connies.
de Havilland DHC-2 Beaver - Neil Aird's site on the world's best and most popular bush plane.
DHC4and5.org - Wayne Buser's website on the DHC-4 Caribou and the DHC-5 Buffalo.
Fairchild C-82 Packet - Simon Beck's website on the venerable Fairchild C-82 Packet. A wealth of information!
Grumman S2F Tracker - David Jackson's excellent website on surviving Grumman Trackers and its derivatives.
AAIR - Aviation Archaeological Investigation and Research - Official U.S. military accidents reports, by Craig Fuller.
Accident-Report.com - Official U.S. military accidents reports, by Mike Stowe.
AeroTransport Data Bank - offers very good info (also on classic aircraft), but for recent info a subscription is required.
Aviation Safety Network - Harro Ranter's website on aircraft accidents since 1943.
NTSB Aviation Accident Database - very extensive accident database since 1982, but for the United States only.
Thai Aviation History - Steve Darke's very well-researched website on Thailand's aviation history.
U.S. Military Aircraft Serials - Joe Baugher's excellent website for researchers.
AirHistory.net Aviation History Image Archive - New aviation photography website.
AussieAirliners.org - Amazing photography from Down-Under by Bob Smith and others. Lots of detailed histories too!
Ed Coates Collection - A huge number of old photographs collected by Ed Coates - with nice captions.
1000 Aircraft Photos - Lots of rare photos & good background information.
ABIX Photo Gallery - Many rare classics on Air-Britain's Information eXchange photo gallery. Can be slow at times.
Planepictures.net - A good aviation photography website.
Jetphotos.net - Another good aviation photography website.
Airliners.net - Another good aviation photography website.
Aviodrome Museum & Theme Park - Wonderful museum, with the Dutch Constellation and the very rare flying DC-2.
Berlin Airlift Historical Foundation - Preserving the memory of the Berlin Airlift by keeping a C-54 and a C-97 in the air.
Buffalo Airways - The Ice Pilots! Commercial operator of classic aircraft in the north of Canada.
Catalina PBY - Operator of the Dutch Catalina PH-PBY, with many pleasure flights offered!
Catalina Society - Catalina preservation in the United Kingdom, and publishers of the Catalina News.
Classic Air Force - Formerly Air Atlantique Classic Flight, a true classics operator. A dying breed in the world...
Coulson Flying Tankers - The former operators of the world's last magnificent Martin Mars flying boats.
DDA Classic Airlines - Formerly Dutch Dakota Association, the first preservers of classic aviation in the Netherlands.
Everts Air Cargo - One of the few remaining commercial operators of classic aircraft. Very friendly people!
FlyingBoxcar.com - John Reffett's and Lars Gleitsmann's site about keeping C-119 Flying Boxcars in the air in Alaska.
Kachemak Air Service - Bill de Creeft's company, still doing sightseeing flights with a 1929-built Travel Air 6000B!
Springbok Classic Air - Flippie Vermeulen serves the tourists in South Africa with his classic DC-3.
Propliner Magazine - The one and only Propliner Magazine, there is no other like it. Simply the best!
AirNet - Howard Curtis' excellent site, with what must be thousands of links to all kinds of aviation websites.