SMath Studio - Tiny, powerful, free mathematical program with WYSIWYG editor and complete units of measurements support.
Blender - Blender is the open source, cross platform suite of tools for 3D creation.
Wolfram Mathematica - Mathematica has characterized the cutting edge in specialized processing—and gave the chief calculation environment to a large number of pioneers, instructors, understudies, and others around the globe.
FreeCAD - An open-source parametric 3D modeler
MATLAB - A high-level language and interactive environment for numerical computation, visualization, and programming
EngineeringPaper.xyz - Online app for engineering calculations. Solves systems of equations, checks and converts units automatically, supports plotting, and allows for the creation of a link to share your work. Free and open source, no registration required.