WARNING: I just found out that BlenderCAM is not available for the latest version of Blender(2.66). The project appears to be dead. Since I'm unable to use it and/or test it I recommend people look at FreeCAD first for 3D modeling because Blender is difficult to use for CAD without all the extra CAD menus.
Linux, Windows and Macintosh
Blender is an open-source 3D modelling program similar to 3D Studio max. BlenderCAD is an extension or skin that you add to it to make it more like a CAD/CAM program.
Here's the download link for the blendercad addons http://sourceforge.net/projects/blendercad/
Blender is a free and open source 3D animation suite. It supports the entirety of the 3D pipeline—modeling, rigging, animation, simulation, rendering, compositing and motion tracking, even video editing and game creation. Advanced users employ Blender’s API for Python scripting to customize the application and write specialized tools; often these are included in Blender’s future releases. Blender is well suited to individuals and small studios who benefit from its unified pipeline and responsive development process. Examples from many Blender-based projects are available in the showcase.