BRL-CAD is a powerful cross-platform Open Source combinatorial Constructive Solid Geometry (CSG) solid modeling system that includes interactive 3D solid geometry editing, high-performance ray-tracing support for rendering and geometric analysis, network-distributed framebuffer support, image and signal-processing tools, path-tracing and photon mapping support for realistic image synthesis, a system performance analysis benchmark suite, an embedded scripting interface, and libraries for robust high-performance geometric representation and analysis.
XCircuit is a program for drawing publishable-quality electrical circuit schematic diagrams and related figures, and produce circuit netlists through schematic capture.
The gEDA project has produced and continues working on a full GPL'd suite and toolkit of Electronic Design Automation tools.
TinyCAD is a program to help you draw circuit diagrams.
Kicad EDA 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.
Open CASCADE Technology is software development platform freely available in open source.