Abandoned & Little-Known Airfields, a site maintained by Paul Freeman, is an extensive and fascinating resource, and was essential for tracking down many of the locations of original manufacturing plants described on this site.
Don’t leave home without checking Mike Henniger’s Aerial Visuals Locator for locally based and displayed vintage airplanes.
Airport-Data is a terrific site for searching recent civil registrations in many countries, whether you are figuring out what happened to an airplane you photographed several years ago or who owns a plane you saw yesterday.
Geoff Goodall’s Aviation Site is a nice warbird site and it contains an indispensable resource in the online, downloadable version of the Warbirds Directory, a compilation of preserved ex-military aircraft that used to be published by Goodall and a co-author in book form. As of this writing, the Directory is up to version 6 and is updated to 2013. Everyone who tracks individual warbirds owes Geoff a big debt for compiling and releasing this database.
Golden Years of Aviation is an invaluable database for researching the original identities of aircraft built in the 1920s and 1930s.
The Internet Movie Plane Database is a fascinating and enjoyable site for browsing or looking up what plane appeared in which movie or TV show.
Researcher and self-described “aircraft serial number freak” Joe Baugher has compiled an epic listing of serial numbers of U.S. military aircraft which is a necessary stop for almost anyone researching the history of an ex-military U.S. aircraft.
Key Publishing’s Aviation Forum is an active gathering place for aviation preservation enthusiasts, especially from the United Kingdom.
Oldprops is a terrific site for researching the identities of surviving propeller-driven airliners.
Warbirds Resource Group is a valuable site maintained by Scott Rose containing the Warbird Registry, a fairly comprehensive listing of preserved World War II and Korean War aircraft, the Warbird Information Exchange, a lively forum where news and views on the subject are exchanged, and other resources.
Planespotting is an amazing site for those who like to look around airports and other aviation venues, not only to spot and log modern aircraft but also for any preserved or derelict planes that may be hanging about.