FreeCAD is an Open Source CAx RAD based on OpenCasCade, Qt and Python. It features some key concepts like macro recording, workbenches, ability to run as a server and dynamically loadable application extensions and it is designed to be platform independent.
Currently, FreeCAD can import and display CAD models in IGES, STEP, and BRep formats and meshes in STL, BMS, AST and Wavefront OBJ formats. Editing and modeling features are currently somewhat limited.
More about FreeCAD: http://sourceforge.net/apps/mediawiki/free-cad/index.php?title=Main_Page
FreeCAD is available in Ubuntu Software Center, but it may be old. Download latest version here