FreeCAD
A free and open source multiplatform 3D parametric modeler
... is a multiplatfom (Windows, Mac and Linux), highly customizable and extensible software. It reads and writes to many open file formats such as STEP, IGES, STL, SVG, DXF, OBJ, IFC, DAE and many others, making it possible to seamlessly integrate it into your workflow. FreeCAD is designed to fit a wide range of uses including product design, mechanical engineering and architecture. Whether you are a hobbyist, a programmer, an experienced CAD user, a student or a teacher, you will feel right at home with FreeCAD.