I find IPE to be a life-savior when it comes to generating physics illustrations which come with their share of math toppings. The editor has nice utilities for latching on to the boundaries, intersection, center etc of geometric figures. The spline tool is the best I have seen anywhere. Layers are handled with grace. LaTeX is rendered instantly. It is very easy to produce a multipage document. What more can one possibly want?
Overleaf - The online platform for scientific writing. Overleaf is free: start writing now with one click. No sign-up required. Great on your iPad.
Inkscape - Inkscape is a free, open source professional vector graphics editor for Windows, Mac OS X and Linux.
TeXstudio - TeXstudio is an integrated environment for writing LaTeX documents.
LaTeXDraw - LaTeXDraw is a graphical drawing editor for LaTeX.
LyX - LyX is a document processor.
draw.io - Online diagramming application