Overleaf - The online platform for scientific writing. Overleaf is free: start writing now with one click. No sign-up required. Great on your iPad.
SMath Studio - Tiny, powerful, free mathematical program with WYSIWYG editor and complete units of measurements support.
TeXstudio - TeXstudio is an integrated environment for writing LaTeX documents.
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.
LyX - LyX is a document processor.
MATLAB - A high-level language and interactive environment for numerical computation, visualization, and programming