MATLAB - A high-level language and interactive environment for numerical computation, visualization, and programming

Overleaf - The online platform for scientific writing. Overleaf is free: start writing now with one click. No sign-up required. Great on your iPad.

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.

TeXworks - The TeXworks project is an effort to build a simple TeX front-end program (working environment)...

Maxima - Maxima is a fairly complete computer algebra system written in Lisp with an emphasis on symbolic computation.

Archimedes - Archimedes is a LaTeX and Markdown editor designed to make mathematical writing easy.