via Fabio Sergio

a Wired article lists a bunch of really cool graffiti hacking projects:
Movable Type in NY - projecting graffiti
Art Bot in Zurich - a robot that can reproduce scanned images
Phone Tag in London - projecting text messages
Paintball Printer in Groningen
Tunnel Visions in Berlin - projecting animations in the subway
Flashlight Brigade in Amsterdam - capturing light sources with long exposure cameras