Drawball is a site that allows anyone to draw whatever they wish on a large virtual "canvas". Each user can only draw a tiny amount per day (approximately equal to 1/10,000th of the surface), so advertisers and online communities often come together to create larger impressions (such as the Mario and Dinosaur above).
Drawball just released a really neat animation showing the evolution of the ball in 40 hour snapshots since its creation. I particularly liked how the South Korean flag morphed (and eventually turned into a Pepsi logo) as users tried to maintain it. Link