As Github is to software, upverter is for schematics/pcbs, so too is OnShape for CAD. Check it out. www.onshape.com/
I know 123D already does some stuff completely in the browser, but OnShape is supposed to be the fully featured CAD of your dreams. Even more interesting, it follows the Github pricing model where you get public documents for free but you have to pay for private documents. Although you do get 5 free private docs to start.
I set up an organization for the MakerHive, hopefully that makes sharing models easy. Though I haven't actually drawn anything more than a rectangle in this yet.
I figure this will either be Github-awesome or Upverter-not-that-great. We'll see.
Bunch of $#!7. Here is my reply : We received your request for an invite to Onshape Beta. Due to exceptionally strong demand, we are still processing requests for Onshape Free accounts on a first-come-first-served basis. We will email you as soon as your free account is ready. We look forward to having you join the Onshape Beta community.