Celt

This is a celt or rattleback - an object that in spite of its simplicity has a preferred direction of rotation when you spin it on a table. Spin it the wrong way and it slows, starts rocking, and often starts spinning the right way.
A demonstration: https://goo.gl/photos/vXfbPJD4R7eT9vmL6
Print Settings
Printer:
Kossel Mini

Rafts:
No

Supports:
No

Resolution:
0.2 mm layer height

Infill:
100%

Notes:
To get the weight distribution right, this object needs 100% (or nearly) infill. Be warned that slic3r does not cope terribly well with filled objects so you can get unsightly lumps. The object also contains voids with overhangs, but the print worked out okay for me.
Post-Printing
The layer structure of a 3D print makes the bottom surface a bit too rough fot it to spin well. A few minutes with a piece of sandpaper smoothed mine enough that it now works well.