Graffiti Archaeology is an interesting site that studies "graffiti-covered walls as they change over time. The grafarc.org project is a timelapse collage, made of photos of San Francisco graffiti taken by many different photographers from 1998 to the present.
"Using the grafarc explorer, you can visit some of San Francisco's classic spots, see what they looked like in the past, and explore how they have changed over the years."
Check it out.
