This is absolutely fantastic and fascinating. Always glad to see the continuing transition of libraries from physical to digital. Also, imagine the potential for a mashup between this and the current/historical Street Views available on Google Maps.

You can read more about this project on its about page: http://www.oldnyc.org/about.html

I love this -- great way to visualize these images.

Also, looks like http://old.nyc is unregistered :)