The 4x4x6 has been mass produced! Get one for just $38 (free shipping!) from Mefferts.com
The first 4x4x6 cuboid was made in 2004 by Tony Fisher, based on an idea of Geert Hellings to modify a 4x4x4 to have two extra layers. Joshua has made a very generous offer and paid me to design a 3D printable 4x4x6, and make it open source.
Joshua's initial proposal was to make the design based on the Rubik's 4x4x4, but I suggested to use a completely new mechanism since it would most likely result in a much higher quality while not raising the cost very much.
This new mechanism works really well and the result puzzle is absolutely perfect. It turns smoothly and even cuts corners some. In the video it makes a quite loud squeaking noise but it has been getting better.
The SolidWorks file for this puzzle, as well as the STL files, are available from thingiverse
under a Creative Commons license.If you order this model, you will get a DIY kit of the plastic parts needed to make this puzzle. You will need to supply your own screws (M3), stickers and springs for assembly.