What are some of the Websites/web-tools which must be a part of every mathematician's arsenal? 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.

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*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.
 A: *

*Overleaf is a good online LaTeX editor, and it particularly nice for collaboration.


*CalcPlot3D is a nice tool for 3D graphing, as is Geogebra, though Geogebra can be a bit more finicky.


*TeXRendr is a simple real-time online TeX renderer.  It's not meant for anything too advanced, and can't be saved, but it's nice if you just want to check that something will display correctly before compiling.
A: Quiver lets you draw commutative diagrams easily and export to $\LaTeX$, and MathPix Snip lets you convert screenshots of equations to $\LaTeX$. Both very useful.
Also, libgen and scihub are indispensable with the modern publishing system...
A: Here is another list that I managed to collect, and was too long to be posted in the comments--

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*Latex to word converter


*Matrix Calculator--Really powerful computing


*number empire--Useful for number theory


*3d math tool


*Pi search page-for pi lovers


*Prime number finder--by numbereducation


*Meta numbers--MetaNumbers is a free math tool providing information about any positive integer (up to 9223372036854775807), such as its factorized form, its divisors, its classification, or its arithmetic properties (widely used in the field of number theory).


*Crazy numbers--Mathematical operations, numerology, symbolic numbers, representations, mathematical games, find everything you've always wanted to know about numbers.


*other tools and calculators  by math is fun


*calculators (for those who cannot code python, these prove very useful sometimes)--
a) Big number calculator--can calculate even 1,00,000 factorial !!
b) Google calculator-Calculate on the go
c) Desmos calculator--Quite powerfull
d) Google sheets-I had used them to generate lots of Random numbers I needed for a complex algorithm and tabular calculation


*truth-table generator;
another one;
this one generates a URL to the required truth table, and also a LaTeX truth table


*tree-proof generator--even provides counterexample
A: You probably want most answers to just give one suggestion, but I'll mention a pair that serve complementary aims. To convert LaTeX to an image, use codecogs; to find out the LaTeX command for something you draw by hand, use detexify (but you have to draw quickly).
A: The Math-aware search engine: Approach Zero is great for online investigations of mathematical expressions.

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*Example: Looking for $[x^n]\frac{1}{\sqrt{1-x}}$ provides this result.

A: Here are some recommendations. I think saying that they must be a part of every mathematician's arsenal is too strong; although I find them useful/interesting/fun:

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*https://www.cs.unm.edu/~joel/NonEuclid/NonEuclid.html to draw shapes and build intuition on the disk model of the hyperbolic plane.


*http://www.numbertheory.org/php/smith.html to compute the Smith normal form of a matrix with integer entries (I used this e.g. here: https://math.stackexchange.com/a/4320501/169085).


*https://jasonwarta.github.io/latex-matrix/ to obtain the $\LaTeX$ code for matrices hastily (I used this e.g. here: https://math.stackexchange.com/a/4367845/169085).


*https://truben.no/latex/bibtex/ to get Bibtex entries.
