One of the best new features released for Google Earth in the past few years is undoubtedly their Historical Imagery. With a click of an icon, you can go back in time and view an image of area that is years (or sometimes decades) ago.
While not yet connected to Google Earth in any way, the new Britain From Above website has over 16,000 aerial images on their site, some going back nearly a century. For example, this 1928 image for the “Metropolitan Cattle Market” in Islington is excellent:
Virtually all of the imagery on their site is oblique (45 degree angle), which is wonderful to look at but means that it isn’t suitable for addition to Google Earth’s Historical Imagery archives. However, I’d love to see them put together a KML file for quick viewing of images by location, similar to the [Panoramio] layer that is already in Google Earth.
Despite that, the site has a great interface and makes it easy to browse through their library of photos. Check it out at BritainFromAbove.co.uk.