Do-Nothing Machine

The Trammel of Archimedes is a device used to trace an elliptical path. It is sometimes made as a novelty and called a "do-nothing machine" or a "smoke grinder." It is extremely easy to print and assemble but hard to put down.

Wikipedia has an awesome .gif of it in action
http://upload.wikimedia.org/wikipedia/commons/9/9f/Trammel_of_Archimedes_Large.gif

The arm could be made more complex, to add a handle or pen holder, but my aim was simplicity. I didn't want to distract from the motion