It's the most fascinating internet time-suck you'll come across today.
OldNYC is a vast collection of historic photos of the city (sourced from the NYPL) mapped to their geographic coordinates.
Created by software engineer Dan Vanderkamp, the interactive map is comments-enabled and Vanderkamp encourages visitors to make contributions to it, such as noting their perspective on the changes that have happened in their neighborhood, correcting inaccuracies in the photo captions or rotating incorrectly oriented images.
Vanderkamp's past projects include OldSF and a geographic area comparison website called Comparea. He notes that there's a lot of activity around the city's bridges on the map, and that comments range from very casual (people commenting "I live here!") to in-depth reports about the images by neighborhood historians. One major difference between the images on the map and present-day New York: street trees. We have many more of them in the present day than we did back then.
Work on the map will be ongoing as Vanderkamp makes updates, which include mapping parts of the city that have been subject to significant structural changes since the archive photos were taken, such as Stuyvesant Town.
He's published the map's source code on github, and looks forward to seeing how others will apply his work. At an NYPL hackathon last year, someone developed a FourSquare integration for an older version of OldNYC; the other possibilities are endless.
Would you recognize your own home circa 1930?