The basic idea of this webpage is quite simple: Take a vintage photo, take an identical photo of the site (with the same angle, etc.) as if the camera never moved and overlay the two. Must have taken a lot of time to get 'em lined up right.
http://ace.methodcomputers.com/ghost/ - via digg
Update 11/6/06: That's North Carolina State University campus - Thanks megmeg and CJ!