SMath Studio - Tiny, powerful, free mathematical program with WYSIWYG editor and complete units of measurements support.
MATLAB - A high-level language and interactive environment for numerical computation, visualization, and programming
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.
C++ - Has imperative, object-oriented and generic programming features, while also providing the facilities for low level memory manipulation
D (Programming Language) - D is a language with C-like syntax and static typing.
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.