Kinekt Design's Gear Ring

The Gear Ring (designed by Ben Hopson and Glen Liberman) is made from high quality matte stainless steel. It features six micro-precision gears that turn in unison when the outer rims are spun (as can be seen in the video). It is currently available for purchase in four standard US ring sizes: 9, 10, 11, and 12.

Clever design.

More info at http://www.kinektdesign.com