I would like to know math websites, that are useful for students, PhD students and researchers (useful in the sense most of the students or researchers -of a particular area- are using it). Maybe you can share which math websites you sometime use and why you use it.
Let me give my websites and why I use them:
MathOverflow and StackExchange: Very good to look up questions about how to do mathematics, what mathematics is, how it evolves, and of course to have an exchange with smart people, who can help you if you stuck at something.
arXiv: I don't use it now, but I think that's the main website to read papers and to publish (research level)
The Stacks Project: It is very useful for me to loop algebraic things ups with more explanation as in some lectures...
MathSciNet: To search for papers, that may help you.
Number Theory Web: Since I want to become a number theorist, this site helps me to see which kind of things exist.
Have you more websites, that help you to understand maybe mathematics, or just to connect to other mathematicians? Everything is welcome, just post it as an answer and not a comment.
Thank you in advance!!