mmorrisson, in a comment on my recent Drawing Games post, pointed to another online drawing game: Linerider. The idea is simple, but with a twist: use the pencil to draw a slope, then "play" it, letting a tiny sled rider ride the slope. It seems simple at first, but getting the sledder to keep moving and not fall off can be tricky. I quickly ran out of ideas for adventures to put the little guy through, but others seem to have no shortage: they've created some remarkable landscapes. YouTube also has movies of virtuoso Linerider performances.
Update: I originally had pointed to a different Linerider site, which was scuzzier, and had questionable content, including possibly viruses. These links are much better. I apologize.