We recently had a visit from a medium-size military cargo plane and I was curious as to what it was. One of the airport kids knew what it was and I used the information to find out more. But it started me thinking about sites that might be useful for identifying aircraft and I found a few.
This Air Force site has lots of useful links if you are into military stuff. It also lists all of the active duty aircraft in the Air Force. Link
For large aircraft Airliners.net is a good source. They have pictures of GA aircraft as well.
Westin’s Classic aviation page has hundreds of classic GA aircraft. It’s a bit hard to use but very interesting. Link
FlightAware has a section of photos in addition to its flight tracking services.
I know they don’t intend for their sites to be used to research aircraft, but I find that some of the sales sites are great for identifying aircraft. They are also good for checking out instrument panel layouts, mods, and paint schemes.
Aerofiles has 6,153 photos of aircraft, along with the history of dozens of aircraft companies.