Free and open-source computer-aided design/What exists already?
There are some two-dimensional (2D) open-source CAD programs such as QCAD but these really only replicate the methods that were used to create hand-drawn paper engineering drawings and lack the advantages and power that modern 3D CAD offers.
BRL-CAD is a 3D CAD package that came out American Military ballistics research and can be traced back to the 1970s. It is based on the older constructive solid geometry 
paradigm that isn't as flexible as the parametric 'B-rep'-based software used by modern proprietary CAD programs, although it is highly suited to physical analysis, which is what it was designed for.
However there is a real need to create (or evolve) a really sophisticated, easy to use, open-source CAD (or more ambitiously product lifecycle management ) application that is highly intuitive to use, powerful, extendible and importantly hooks in to what should be a rapidly growing ecosystem of designs and physical systems available to all in the 'universal commons'.
FreeCAD is designed to be a feature based parametric CAD modeller and CAE platform, employing the OpenCASCADE geometric modelling kernel libraries. Extensions framework and integrated Python interpreter for powerful scripting capabilities. No sketcher module yet.
HeeksCAD is a parametric modeller initially aimed at designed parts for CNC machining. Also based on OpenCascade. This has the ability to create 3D parts from 2D sketches with the GUI.
The following are capable 3D modelling programs that although they are designed for graphics and animation, it is possible to use them to create parts for physical designs. However they do not possess many features required for sophisticated mechanical CAD design.
Blender is a powerful 3D modelling, animation and rendering program designed primarily to create sophisticated computer graphics rather than being tailored for design and engineering. It is an advanced and capable piece of software, and might well form a good basis for a dedicated open-source CAD application – such are the possibilities when the software is open-source. There have been some attempts to create scripts extending Blender in this direction, but they are either no longer active or do not yet go far enough. The user interface is not known for being particularly intuitive, however those that know their way around it (including the many vital keyboard shortcuts) say it is very effective.
Art of Illusion is another interesting open-source 3D modelling and rendering application, although perhaps not quite as advanced as Blender. However the user interface is much more intuitive. Similarly it is designed for creating computer graphics rather than as a CAD program, but the required features could be added. The RepRap open-source 3D printer project are currently using this for their 3D modelling with some additional scripting.
