Line Rider is not quite a game but not quite a toy or drawing tool either. But judging by the 6,000,000 views its gotten since it was posted a month ago, Line Rider certainly is compelling. I don't even like playing it all that much, but I spent a solid hour a few weeks ago watching videos of other people's tracks on YouTube; it's just so fascinating to see how much can be done with simple lines and rules. Here's one of the better tracks I've seen (c/o clusterflock). This little non-game has even shown up in Time magazine. Go, little Line Rider, go.
Update: A new version of Line Rider is to be released soon. New features will include an eraser, new types of lines, line snapping, etc.
If you're looking to record your Line Rider creation and post it to YouTube, you can use CamStudio (Win), Super Screen Recorder (Win), oRipa Screen Recorder (Win), Screen Movie Recorder (Mac), iShowU (Mac), Snapz Pro (Mac), and ScreenRecord (Mac).
For information on how to play Line Rider more effectively, check out the Line Rider Forums.