Edit: I had some trouble with this originally and VeryWetPaint was nice enough to fix some things and uploaded a derivative. I didn't want to leave this half finished though so now that I've printed and tested, I'm updating with new models and the original Inventor files. If I get a chance I may try to figure out how to model this in Openscad.

A fun and confusing toy that will rotate freely in one directly, but when spun the opposite direction will stop, wobble, and reverse directions.

More on wikipedia: http://en.wikipedia.org/wiki/Rattleback

Uploaded at the request of Spacexula

You can print one or both files. They are mirror images and differ only in the direction of preferred spin. Print out with a heavy (preferably 100%) infill for best effect. Sand them down until they are smooth along the axis of the long curve (note that is at an angle to the centerline of the part).

The amount of friction between the toy and surface can be the difference between reversing for only a few degrees or a few full turns so try them out on different tables, floors, etc.