The Single Lane Superhighway is a simple website that lets you draw a car and join the parade of other drawn cars already on the road. Simple, yet sweetly engaging. Another interesting art project from Aaron Koblin, who has done several other notable social/data art projects (some of this, some of that, and some other things). Via waxy.
To celebrate its tenth birthday, popular site DeviantART unveils Muro, a gorgeous HTML5 drawing tool that handles multiple layers and a variety of artistic brushes. No account required.