KiCad - A Cross Platform and Open Source Electronics Design Automation Suite
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...
SketchUp - 3D for Everyone
QUCS - Qucs, briefly for Quite Universal Circuit Simulator, is an integrated circuit simulator which means you are able to setup a circuit with a graphical user interface (GUI) and simulate the large-signal, small-signal and noise behaviour of the circuit.
LibreCAD - An open source 2D CAD application for Windows, Apple and Linux.