A new resource mentioned in this morning's New York Times allows you to look at a map of NYC from above with a satellite view. Of course, that's simply Google Earth, but there's a little bar in the interface that you can move to see the same view in 2006...and in 1924. It's kind of neat to be able to do what's essentially a Google Earth satellite view for 1924, and also very useful to historians. Here's the link:

http://gis.nyc.gov/doitt/nycitymap/

To make it work, you need to do a close enough view and to press the little camera button. The interface is somewhat slow, but the map is still fun to look at.