OpenSCAD is a software for creating solid 3D CAD objects. It is free software and available for GNU/Linux, MS Windows and Apple OS X.
Unlike most free software for creating 3D models (such as the well-known application Blender), OpenSCAD does not focus on the artistic aspects of 3D modelling, but instead focuses on the CAD aspects. So it might be the application you are looking for when you are planning to create 3D models of machine parts, but probably is not what you are looking for when you are more interested in creating computer-animated movies.
OpenSCAD is not an interactive modeller. Instead it is something like a 3D interpreter that reads in a script file that describes the object and renders the 3D model from the script file. This gives you (the designer) full control over the modelling process and enables you to easily change any step in the modelling process, or even to produce designs that are defined by configurable parameters.
OpenSCAD provides two main modelling techniques: First, constructive solid geometry (CSG) and second, extrusion of 2D outlines. Autocad DXF files are used as the data exchange format for the 2D outlines. In addition to 2D paths for extrusion, it is also possible to read design parameters from DXF files. In addition to reading DXF files, OpenSCAD can also read and create 3D models in the STL and OFF file formats.
OpenSCAD can be downloaded from http://openscad.org/. You may find extra information in the mailing list.