Global Mosiac

http://www.thebroth.com/ has a project called the Global Mosaic -
a bunch of users from anywhere in the world log in and move tiny tiles to form pictures. It's ever-changing, and as the technology used is AJAX, it is instantly updated and appears as a real time animation.
I stop short of saying it's addictive, since you really can't make great pictures (or at least I can't) with tiles this way, and also people keep wrecking whatever you make.