Aerial map of NYC from 1924 FEB 01 2010
The interactive map on the NYC govt site has hi-resolution aerial photos from 1924 (click the camera and move the slider to 1924). Check out all the piers, the Brooklyn Navy Yard, the old baseball stadiums, the LES (and everywhere else they built housing projects), Penn Station, and the skyscraperless Midtown. This is hours of fun.
Update: The NYC Oasis map features a satellite view from 1996 and an imagined sat view from 1609. (thx, steve)