The following is a list of some links that I find useful. These are mostly mathematical and related to topics that are of some use to mathematicians.
- SageMath: This is a wonderful and extremely powerful open source mathematical software system. It is specially strong and well suited for doing computations and experiments in number theory.
- MathSciNet: A database of the American Mathematical Society containing reviews and abstracts of mathematical publications.
- arXiv: A repository of preprints in mathematics and other scientific fields. This links to the recent submissions in number theory, since that's what I'm most interested in.
- MathOverflow: A forum for research level mathematics.
- Mathematics Stack Exchange: A forum for mathematics of any level.
- TeX-LaTeX Stack Exchange: A forum for users of the typesetting systems TeX and LaTeX.
- MacTutor History of Mathematics archive: A webpage containing lots of history and biographies of mathematicians.