123D  is a hobbyist CAD and modelling tool created by Autodesk. It is similar in scope to Trimble SketchUp and based upon Autodesk's Inventor product range. As well as the more basic drawing and modelling capabilities it also has assembly and constraint support and STL export. Available for the software is also a library of ready-made blocks and objects. Autodesk is also working in collaboration with three companies (Ponoko, Techshop and 3D Systems) to enable users of 123D to create physical objects from their designs using 3D printing technology. Freely downloadable.
Windows, MacOSX, IOS