elmer - Elmer is an open-source software package that solves partial differential equations. Combined with its ability to run various equations in parallel, it is an effective tool for running multiphysical simulations. Read more about elmer.
FreeCAD - An open-source parametric 3D modeler
COMSOL Multiphysics - COMSOL is the developer of COMSOL Multiphysics software, an interactive environment for modeling and simulating scientific and engineering problems.
SketchUp - 3D for Everyone
SimFlow - SimFlow is a powerful CFD software, focused on engineering and science.
LibreCAD - An open source 2D CAD application for Windows, Apple and Linux.