SOLVESPACE is a free (GPLv3) parametric 3d CAD tool. Applications
include:

 - modeling 3d parts - draw with extrudes, revolves, helixes and
    Boolean (union / difference / intersection) operations
 - modeling 2d parts - draw the part as a single section, and export
    DXF, PDF, SVG; use 3d assembly to verify fit
 - 3d-printed parts - export the STL or other triangle mesh expected
    by most 3d printers
 - preparing CAM data - export 2d vector art for a waterjet machine
    or laser cutter; or generate STEP or STL, for import into
    third-party CAM software for machining
 - mechanism design - use the constraint solver to simulate planar
    or spatial linkages, with pin, ball, or slide joints
 - plane and solid geometry - replace hand-solved trigonometry
    and spreadsheets with a live dimensioned drawing

