OpenSCAD

For 3D modelling you may want to look into openscad (at openscad.org) written by Clifford Wold (you met him @metalab, he fixed your power cord). This uses CSG constructive solid geometry http://en.wikipedia.org/wiki/Constructive_solid_geometry) internally and can generate STL but also *import* STL (and use CSG on the resulting model). It also can read 2D Designs in DXF and extrude parts of it into the 3rd dimension (extruding can be linear or on different paths, so you can make, e.g., a torus by extruding a circle on a circular path).