Autodesk 123D

is a hobbyist CAD

Computer-aided design

Computer-aided design , also known as computer-aided design and drafting , is the use of computer technology for the process of design and design-documentation. Computer Aided Drafting describes the process of drafting with a computer...

 and modelling tool created by Autodesk. It is similar in scope to Google

Google

Google Inc. is an American multinational public corporation invested in Internet search, cloud computing, and advertising technologies. Google hosts and develops a number of Internet-based services and products, and generates profit primarily from advertising through its AdWords program...

 Sketchup

SketchUp

SketchUp is a 3D modeling program marketed by Google and designed for architectural, civil, and mechanical engineers as well as filmmakers, game developers,...

 and based upon Autodesk's Inventor

Autodesk Inventor

Autodesk Inventor, developed by U.S.-based software company Autodesk, is 3D mechanical solid modeling design software for creating 3D digital prototypes used in the design, visualization and simulation of products...

 product range.

As well as the more basic drawing and modelling capabilities it also has assembly and constraint support and STL

STL (file format)

STL is a file format native to the stereolithography CAD software created by 3D Systems. This file format is supported by many other software packages; it is widely used for rapid prototyping and computer-aided manufacturing. STL files describe only the surface geometry of a three dimensional...

 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

Ponoko

Ponoko is an online service for manufacturing. It gained some considerable media attention because of its unique business model. Ponoko is one of the first manufacturers that uses distributed manufacturing and on-demand manufacturing.-Distribution Model:...

, Techshop

TechShop

TechShop is a chain of member-based workshops that lets people of all skill levels come in and use industrial tools and equipment to build their own projects...

 and 3D systems)

to enable users of 123D to create physical objects from their designs using 3D printing

3D printing

3D printing is a form of additive manufacturing technology where a three dimensional object is created by laying down successive layers of material. 3D printers are generally faster, more affordable, and easier to use than other additive manufacturing technologies. However, the term 3D printing is...

technology.
It is currently in public beta and freely downloadable.