Utility Websites
These websites offer powerful tools, useful information, or are downright convenient.
Latex Companions

Detexify. Search for Latex commands by handwriting symbols. The description of the command includes the package (if nonvanilla) and whether it should be used in math mode or text mode.

Overleaf is great for collaborative work on LaTeX documents. You can share with more than one person for free if you create a share link, rather than sharing directly.

Quiver and Tikzcd are sites for creating mathematical (commutative) diagrams and exporting them in LaTeX code.

MathPix can be used to convert images/handwriting into LaTeX code. Itâ€™s far from perfect, though, and personally, I find it easier/faster to type directly especially when the formulae are very specialized/precise.
Online Graphing
 Geogebra is useful for graphing and creating geometric diagrams through elementary constructions. It also has a 3D graphing calculator.
 Desmos is simpler to use for ordinary 2D graphing and adding parameters, though Geogebra has the same features and also the geometry and 3D modes.
Scholarship

MathSciNet and Google Scholar are very useful for creating BibTex code for references. I tend to find the latter more useful, as it is both free/does not require university login to access and creates better short names for the references.

Arxiv and its French counterpart https://cnrs.hal.science/ are exceedingly useful for finding openly available scientific articles and/or preprints.
Organization
 When2Meet. Find overlapping scheduling times. One person must create the poll at the provided link by choosing the dates (by default, the time windows are set from 95, but this may also be changed.) Once made, a link to the poll is created. Here is an example poll, where I have indiciated one person to be available on MWF from 1012AM and TR from 13PM. As more people fill out the poll, the schedule on the left will change; the brightest green indicates where the overlap occurs, and the more faded sections indicate less overlap. White indicates no overlap.