Google Maps includes arial/satellite photography. Because GoogleMaps makes it so easy to scroll around, you can stumble across some interseting stuff.
I just stumbled across this plane in flight about to land at the airport in Boise, ID.
Turns out others have spotted airplanes in flight using Google Maps. The previous link is from http://www.googlesightseeing.com, a site dedicated to the cool things people find using Google Maps.