FreeCAD - An open-source parametric 3D modeler
Proteus PCB design - Proteus PCB design combines the schematic capture and ARES PCB layout programs to provide a...
LibreCAD - An open source 2D CAD application for Windows, Apple and Linux.
upverter - Upverter makes it easy to design and share electronics on the web.
SketchUp - 3D for Everyone
Ktechlab - KTechLab is an IDE for microcontrollers and electronics.