Laser Tag

Take a handful of international graffiti artists, an abandoned building in Rotterdam, a smattering of hot tech (a super-high-intensity 60-milliwatt green laser pointer, a 5,000-lumen DLP projector and a sensitive security camera) all crammed into an old RV; add a sprinkle of the DIY spirit, and what do you get? The coolest thing we saw on the Internet today, that's what.

Basically, the folks at Graffiti Research Lab ("Dedicated to outfitting graffiti artists with open source technologies for urban communication." Hmm, sound familiar?) cooked up the world's largest Etch-a-Sketch. Only instead of twisting those little knobs, you write on an 18-story building with a laser pointer that's capable of popping balloons and igniting match heads on contact. Their whole setup is fully documented (along with the source code to the software they wrote).

Basically, a sensitive security camera and a powerful DLP projector are both trained on the facade of a building. A laptop processes the camera's output and indicates the presence of the laser's cursor, which is then fed to the projector and onto the building in the form of a streak of "paint," all in real time. It looks too fun for words.
Check the amazing video of the whole setup in action.