FreeCAD is a parametric 3D modeler. Parametric modeling allows you to easily modify your design by going back into your model history and changing its parameters. FreeCAD is open source (LGPL license) and completely modular, allowing for very advanced extension and customization.
There are several great things about this.
- It’s free as in beer
- It’s free as in libre
- It’s available for Windows, Linux and Mac OS X
- It’s scriptable in python
- you can draw things using constructive solid geometry (CSG)
- you can create objects by extruding 2D drawings similarly to Solidworks