Buy at http://www.shapeways.com/model/34027/cross_rings.html.
Copyright (c) Bram Cohen & Oskar van Deventer. This puzzle ring is a design by Bram Cohen. Bram writes: "It has basically a single trick, but that trick is so cute and understandable that I think it's a compelling puzzle.".This version can be laid flat like handcuffs.
More info at Shapeways Forum