These are links to some online tools and resources that I find useful from time to time and recommend to whoever has not yet heard of them.
A comprehensive list of (math) journals and their abbreviations (pdf): http://msc2010.org/MSC2010-CD/extras/serials.pdf
Citation generator (including but not restricted to DOI -> BibTeX): https://quillbot.com/citation-generator
Online visual editor that can generate a tikz code of the picture: https://www.mathcha.io/editor
The Grammar According to West (a lot of nice advice on mathematical English writing): https://dwest.web.illinois.edu/grammar.html
Kahubi is an AI-equipped note-taking online tool, and the feature I mostly use it for is generating Python code from a text prompt: https://app.kahubi.com/
Another AI-equipped note-taking and PDF-annotating online tool: https://app.avidnote.com/
A simple online visual tool for working with graphs, easy to try some classic algorithms, highly recommend for those doing their first course in graph theory to play around and learn: https://graphonline.ru/