BRL-CAD
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
XCircuit is a program for drawing publishable-quality electrical circuit schematic diagrams and related figures, and produce circuit netlists through schematic capture.
gEDA
The gEDA project has produced and continues working on a full GPL'd suite and toolkit of Electronic Design Automation tools.
Sailcut CAD
TinyCAD
TinyCAD is a program to help you draw circuit diagrams.
KiCad EDA
Kicad EDA is an open source (GPL) software for the creation of electronic schematic diagrams and printed circuit board artwork.
Qucs
Qucs is a circuit simulator with graphical user interface.
Open CASCADE
Open CASCADE Technology is software development platform freely available in open source.