The BRL-CAD package is a powerful Constructive Solid Geometry (CSG) solid modeling system with over 20 years development and production use by the U.S. military. BRL-CAD includes an interactive geometry editor, parallel ray-tracing support for rendering and geometric analysis, path-tracing for realistic image synthesis, network distributed framebuffer support, image-processing and signal-processing tools. The entire package is distributed in source code form.

XCircuit is a UNIX/X11 (and now Windows, if you have an X-Server running) program for drawing publishable-quality electrical circuit schematic diagrams and related figures, and produce circuit netlists through schematic capture. XCircuit regards circuits as inherently hierarchical, and writes both hierarchical PostScript output and hierarchical SPICE netlists. Circuit components are saved in and retrieved from libraries which are fully editable. XCircuit does not separate artistic expression from circuit drawing; it maintains flexiblity in style without compromising the power of schematic capture.

The gEDA project has produced and continues working on a full GPL'd suite of Electronic Design Automation tools. These tools are used for electrical circuit design, schematic capture, simulation, prototyping, and production. Currently, the gEDA project offers a mature suite of free software applications for electronics design, including schematic capture, attribute management, bill of materials (BOM) generation, netlisting into over 20 netlist formats, analog and digital simulation, and printed circuit board (PCB) layout.

Sailcut CAD is a sail design and plotting software which allows you to design and visualise your own sail and compute the accurate development of all panels in flat sheets. The original Sailcut was a Basic program developped by Robert Lainé in 1978. The new version, called Sailcut CAD, is written in C++ and is developped jointly by Robert Lainé and Jeremy Lainé. It is distributed under the terms of the GNU Public License (GPL)

TinyCAD is a program to help you draw circuit diagrams. It comes complete with symbol libraries to get you started straight away. As well as being able to simply print your designs, you can use TinyCAD to publish your drawings by copying and pasting into a Word document, or saving as a PNG bitmap for the web.

Kicad is an open source (GPL) software for the creation of electronic schematic diagrams and printed circuit board artwork.

Qucs is a circuit simulator with graphical user interface. The software aims to support all kinds of circuit simulation types, e.g. DC, AC, S-parameter, Transient, Noise and Harmonic Balance analysis.