Historypin is probably my favorite service for locating geo-located historical imagery. After writing my post about Views of Venice I explored Historypin for the first time in a few months and enjoyed exploring some of the Historypin channels and tours of geolocated imagery.
Some of the Historypin channels of note for teachers of US History are those of the Library of Congress, The Smithsonian, and Historic New England. A personal favorite of mine, although it doesn't have broad appeal, is the Hagley Museum and Library that my grandfather used to take me to as a child in Delaware. On all of the Historypin channels you will find, at a minimum, galleries of historical images geolocated onto Google Maps. Many of the channels also include a tour that you can flip through to view connected image sets.
Applications for Education
Historypin is largely a crowd-sourced effort. Your students could contribute to an existing Historypin project or create projects of their own. Historypin allows anyone with a Google account to place images within the setting of current Google Maps Streetview imagery. Your students could create a Historypin project of their own by going to your local history society, scanning historical images, and placing them into a map. (Try using the CamScanner app for iOS and Android to scan images without having to use a dedicated scanner).