Surprise: Valve has released the CAD files for the Steam Controller, allowing anybody to create accessories or modifications to its hardware. There's not really a catch, either -- they're being released under a Creative Commons license, and Valve asks that you contact them only if you plan to sell your design.
The files contain the info for only the external components of the controller, not its guts.
You can read a few more details and download the ZIP with the files over at Valve's blog.
