Earlier this week Google released "historical" imagery for Google Maps Street View. That imagery only goes back seven years. To show your students historical imagery from the 20th Century, take a look at the following options.
Historypin is probably my favorite service for locating geo-located historical imagery. Historypin allows anyone with a Google account to place images within the setting of current Google Maps Streetview imagery. If you don't have images to add, you can simply explore the imagery added by others. To explore the imagery on Historypin, zoom in on a location then select a range of dates on the Historypin timeline. Learn more about Historypin in the video below.
Historvius is a neat site that is part search engine, part map, and part reference source. The purpose of Historvius is to help people locate historically important sites associated with people and events from 4000BCE through today. Visitors to Historvius can search for places by selecting range of dates, choosing a historic period, selecting a person, or by entering a combination of search terms. Search results are displayed on a Google Map. You can then click on a placemark on the map to find more information about that historic site.
What Was There is a good example of using Google Maps to show the way places used to look. What Was There allows registered users to upload old images of buildings and landmarks and have them displayed in the place where they were taken. Anyone can explore the map and view the historical images on the map. Enter a city name in the search box or browse the map's placemarks to find images. Click on a placemark to open an image then choose "Google Street View" to see the image placed over current image of a place.