FutureLab has a nifty service that erases people, cars, etc. from photographs. It works by comparing several photos of a scene, and getting rid of the stuff that's different from the other photos. Check out the gallery of "ghost town" photos! Link (thanks, Marc!)
Reader comment: moon_custafer says:
You can use this to evoke the photography of the 1840s, when the exposure time was too long for passers-by to register! (as seen in the following photo of Nelson's Column under construction.