For today's post I'd like to highlight the Serge A. Sauer Map Library. Housed at the University of Western Ontario in Canada, the Map Library has large repository of maps in both hardcopy and digital form. The library also has a large stock of aerial photography, and recently made an air photo project flown in 1922 available online. The project page allows you to view and download the photographs via a mosaic or through a Google Maps interface. From the drop-down menu on the Google page, it looks like aerial sets from other years are coming soon.
The steps used to produce the mosaic are also available. While the process was very "manual" (rotating and scaling the images until the fit a basemap), the end result is a good looking historical mosaic!