The NASA Dryden Flight Research Center as a photo page http://www.dfrc.nasa.gov/Gallery/Photo/index.html with hundreds of high quality pictures of NASA aircraft ranging from the 1946 X1 to 2001 Space shuttle flight. Pictures are well indexed and come in multiple resolutions ranging from thumbnails to 3000x2750 Jpeg images (that's several MB). Each picture includes a description of the aircraft, its mission and the associated research program.
Looking for modern military aircraft? Check out the Air Force at http://www.af.mil/photos. The site has a number of quality pictures (generally around 1214x800 jpegs) sorted by different categories such as Bombers, Fighters, Helicopters and so on. Each of these pages comes with thumbnails.
About 40 good pictures of military aircraft including ones from other countries can be found at http://www.cs.ruu.nl/pub/AIRCRAFT-IMAGES/.
A very comprehensive collection of aircraft of NATO countries can be found at http://www.vogue-web.ch/phantom. There are over 600 images indexed with fine thumbnails. The images themselves have good resolution for on-screen viewing.
Edwards AFB has a number of pages of aircraft pictures. Start with http://www.edwards.af.mil/gallery/index.html which links to pages with different classifications. Pictures range from a 1929 P-12 to 2000 F-22
At http://www.wpafb.af.mil/museum/ind/ind.htm you can find an index to the collection of the Air force Museum at Wright-Patterson AFB. The pictures are not very high resolution but have detailed descriptions.
Want more war birds? There are quite a number of them at http://www.mucheswarbirds.com/images.html. They include some description but the resolution is not stellar.
You know there are lots of interesting aircraft at the Smithsonian air and space museum. Well, they're all online. Starting at http://www.nasm.edu/nasm/aero/aircraft/aero.htm you can search for a particular aircraft or click on to one of several index pages that contain alphabetical listings: A - C, D - F, G - K, I - M and N - Z. The images of generally around 640x399 pixels, okay for viewing but not for printing. They also contain a distracting water mark. But there are extensive descriptions of each aircraft and whatever made them famous. So if you are curious how much fuel Voyager used for its around the world trip, here is the place to find out.
A reasonable collection of pictures can be found at the French air and space museum. The site has recently (2003) been reorganized and I have not looked at the new site in detail http://www.mae.org/main.htm.
http://acro.harvard.edu/ACRO/acro_images.html has many thumbnailed images of aerobatic aircraft. The actual images are from various sources and are of varying image quality ranging from just okay to excellent.
Jack Wolbrink has a page http://avia-dejavu.net/Ultralight.htm with European ultralight and home-built aircraft.
Fans of antique and classic aircraft should check out http://www.geocities.com/CapeCanaveral/Lab/4515/Gallery.html you'll find five index pages of thumbnails. The images are generally 640x480. On this same site there is a large collection of pictures from the 1931/1932 Cleveland Air Races, http://WWW.geocities.com/CapeCanaveral/Lab/4515/Cleveland.htm. The pictures are mostly shots of aircraft on the ground. Given that these photos are 70 years old, they are of pretty good quality. Lots of nice pictures with technical specs of historic aircraft can be found at the Golden Age of Aviation eZine.
A somewhat unusual site is http://www.aviationpics.de. You'll find amazing pictures of accidents, mishaps, and their aftermath.
This site http://www.luftfahrtmuseum.com/htmi/general/i.htm has a pretty comprehensive set of links to aviation museums in Europe. They list what's to see and give technical specs, some pictures and some drawings of the exhibits.
And if all of this is not enough for you, check out http://www.landings.com/_landings/pages/images.html which contains links to many more sites with aircraft pictures.