Blender - Blender is the open source, cross platform suite of tools for 3D creation.
KiCad - A Cross Platform and Open Source Electronics Design Automation Suite
SelfCAD - SelfCAD is an online 3D modeling and slicing software.
Fritzing - Fritzing is an open-source initiative to support designers, artists, researchers and hobbyists to...
SketchUp - 3D for Everyone
LTspice - LTspice® is a high performance SPICE simulation software, schematic capture and waveform viewer with enhancements and models for easing the simulation of analog circuits.