LibreCAD - An open source 2D CAD application for Windows, Apple and Linux.
myOpenLab - MyOpenLab is a platform for finished elements (program components) that can be linked together.
FreeCAD - An open-source parametric 3D modeler
PyLab_Works - PyLab_Works is a free and open source replacement for LabView + MatLab, written in pure Python.
SketchUp - 3D for Everyone
LabVIEW - LabVIEW offers many of the functions you'd find in a traditional script-based programming language presented in a more intuitive visual interface ideal fo engineering.