Improved Trammel of Archimedes (aka Ellipsograph or Nothing Grinder)
This is a classic toy that demonstrates some interesting principles of geometry and mechanical motion. It is sometimes called an "ellipsograph" because it traces a perfect ellipse.
Here is a detailed description from Wikipedia with an animation of the "grinder" in action:
http://en.wikipedia.org/wiki/Trammel_of_Archimedes
This is my own design, but it mirrors many well known versions that have fascinated kids (and adults) for thousands of years.
Note that the handle has a hexagonal hole that allows you to insert a cut-down pencil for ellipse drawing.
The motion is hypnotic and I hope you have fun printing and playing with this version of the Nothing Grinder.
Note: This model requires no supports. If you can print with two different filaments then color contrasting the rods and handle lettering can make the model even more intriguing.