Brief list of useful online tools for #LaTeX users:
https://www.unicodeit.net to use Maths symbols on pages that do not support LaTeX (e.g. here)
https://www.tablesgenerator.com/latex_tables to generate tables without going mad
https://editor.codecogs.com to get an image of your equations you can copy-paste
https://detexify.kirelabs.org/classify.html to find what is the command for that less-used symbol you can never remember
https://www.doi2bib.org to get a well-formatted revtex bibliography from the doi of the paper
Do you have more to add? 😀