Also called the Trammel of Archimedes, the "do nothing machine" excels at nothing in particular, but looks damn cool doing it. Now you can build your own!
Instructables member "perry112358" provides the DIY instructions today, using a bit of wood, metal and various workshop tools. Enjoy!
And, yes, the "do nothing" part is actually a misnomer. If you wish, the device serves as a drawing tool called an ellipsograph, should that kind of behaviour strike your fancy. [Instructables via MAKE]