FoxAlpha
Mother site of Picavia, air photography portal
France - english speaking - 4 websites

Helimage
Sister site of Picavia, dedicated to helicopters
France - english speaking

La Ligne par l'Image
A great website, displaying a lot of pictures taken by a french commercial pilot
France - french speaking

KeroZen
French spotting association's website.
France - french & english speaking - more than 5 000 pics

Airliners
An incredible database of commercial and military planes pictures.
Sweden - english speaking - digital library

Planepictures
Huge database of commercial aviation pictures.
Germany - english speaking - digital library

Royal Air Force
UK - english speaking - digital library for subscribed members

United States Navy
USA - english speaking - digital library

United States Air Force
USA - english speaking - digital library

United States Marine Corps
USA - english speaking - digital library

Aviation-fr.info - L'encyclopédie de l'aviation
France - french speaking - more than 250 pages

Pégase
Franck Cabrol's website for warbirds and modern military planes enthousiasts.
Wallpapers - french & english speaking, more than 2 000 pics

Wingshots - aerial photography
Antoine Grondeau's website with aircraft walkarounds and photographs picturing all sorts of flying machines.
France - english speaking - more than 5 000 pics