For Tinkercad users they are rolling out the closure in stages:
Effective immediately they have closed sign-ups for new users
April 30 2013 - All free accounts will be changed to read only
August 31 2013 - All academic accounts will be changed to read only
December 31 2013 - All paid accounts will be changed to read only
June 31 2014 - Read only access for all users will be discontinued
This means if you currently have files stored on Tinkercad, you will have until June 31 2014 to download them from their storage and/or upload them to another repository such as Shapeways, Sketchfab or Thingiverse. If you have unfinished models in Tinkercad you have a limited time to make the modifications to export and/or 3D print them.
I can't think of many science or engineering software companies that haven't had the goal of putting their software tools right on the desktops of users for most practical work. In addition many of them have developed multiprocessor capabilities that can run within a computer or across a network to achieve the gains possible with parallel computing techniques (such as Matlab). As I understand it the TinkerCad (now Airstone) servers have been developed to allow massively parallel processing capability using many thousands of processors to enable near real-time analyses of whatever technical geeks might need to analyze that would be beyond the abilities of locally sited computing resources. It probably makes more financial sense to charge corporations and government for computer processing time to do complex simulations than it does to sell subscriptions to individuals and educational institutions to run a rather rudimentary CAD program. Of course this is just conjecture!
It will be sad to see TinkerCad go since it was fun to introduce people from the ages of 5 to 80 to basic 3D modelling using a laptop and then hit a few buttons to have a bobble manufactured for them. Thanks for the memories!
Try http://www.publishyourdesign.com/modeler, a browser based 3D modeling tool, which provides similar functions as TinkerCad, and perhaps more. Beside the capability to build models using pre-define shapes, the web app is also able to create shapes from 2D sketch using advanced modeling tools extrude, revolve, sweep and boolean operations. It works offline. You could save model in local and save to cloud when has network access later. Model can be exported as STL for 3D printing as well.