LibreCAD - An open source 2D CAD application for Windows, Apple and Linux.
GNU Octave - GNU Octave is a programming language for scientific computing.
FreeCAD - An open-source parametric 3D modeler
Xoctave - The Affordable Matlab alternative which makes you save money. Xoctave is a powerful software that is meant to unleash the power of GNU Octave by creating a complete scientific computing environment.
SketchUp - 3D for Everyone
QtOctave - QtOctave: a front-end for Octave/un Front-End para Octave