Downloadable hack: a 3-D representation of the former World Trade Center towers for Google Earth. See what the towers looked like in actuality, from any perspective or distance, from any part of the city, on the ground or in the air. I know Matt has wondered what they realistically looked like up close. You must already have Google Earth installed.
See also:
http://www.northernlight.net/
I’m seriously doing all my vacation planning on Google Earth this year. It is simply awesome.