Line Rider is, according to the author, not a game but a toy.

Basically you take a pencil and draw curves and lines on a white background. You can then press play and it launches a guy on a sled into your line drawing. The sled slides down the lines.
If you make them too steep the guy crashes. If you make them too flat he just comes to a hault. If you make them just right you can get a crazy roller coaster going on.
One of the most original and addictive flash games ever created. Enjoy!