WhatWasHere is a project that uploads historic photographs to Google Maps so you can see what a location looked like in the past. Photos can be uploaded through the WhatWasHere website or via an iPhone app.
The success of this online project depends on people's willingness to prepare and upload photos. The site tries to make it easy by requiring people to enter in only the location and the year of the photograph. But it will need a lot of participants and a lot of photographs to make it work. If you have a few boxes of old photos kicking around, do your part and start scanning and uploading. [WhatWasHere via Laughing Squid; iTunes link]