Historypin is a fascinating new app that lets you share historical photos by "pinning" them to a world map. You can search for photos by location and through time and compare them to Google Street View shots of the present day. Want to see a picture of city hall in 1913? Or how the conference centre looked when it was a train station 50 years ago?
It's still in beta, and most of the images appear to be from the UK, but I'm looking forward to participating in this exciting project. Check out the short video below.