What are some of the web-tools which must be a part of every mathematician's arsenal? Tools which help mathematicians research, solve problems or even collaborate together!

Here is a list I gathered. Many among you may already know these, But still, in case.

MathQuill-- This is a great website for easy typing in LaTeX.

Integral calculator and the Derivative calculator-- Especially useful when one wants to crack a tough indefinite question. Steps included for those who need them!

OEIS--A list of all mathematically interesting sequences that contains over 3 million of such. Many things are discovered thanks to OEIS.

Desmos--One of the best online graphing tool with loads of computing power.

WolframAlpha and Mathematica-- Who doesn't know them! A unique engine for computing answers and providing knowledge.

MATLAB--Math. Graphics. Programming. Whether you’re analyzing data, developing algorithms, or creating models, MATLAB is designed for way you think and the work you do.

Software such as Maple, MATLAB, Mathcad, Pari/GP, R, and the WolframAlpha website, etc are famous, and this question is not asking for similar software alternatives.

I haven't added math stack exchange to this list for obvious reasons :~)

Thanks in advance to all for the over-enthusiastic help :-)

You might want to check out Online tools for doing symbolic mathematics

PS-No intention of advertisement or promotion of any website or software.