Below are links, grouped into categories, to sites that I find useful and/or interesting.
- arXiv - a repository of electronic preprints (known as e-prints) approved for publication after moderation;
- Google Scholar - a search engine for scholarly literature;
- AustMS/ARC Journal Rankings;
- MSC - 2010 Mathematics Subject Classification Code Database;
- OEIS - the online enclyclopedia of integer sequences;
- ORCID - provides persistent digital identifiers to distinguish researchers from each other;
- GAP - a system for computational discrete algebra;
- GAP semigroups package - a GAP package containing methods for semigroups, monoids, and inverse semigroups, principally of transformations, partial permutations, bipartitions, subsemigroups of regular Rees $0$-matrix semigroups, free inverse semigroups, and free bands;
- Math Overflow - question based (research level);
- Mathematics Stack Exchange - question based (general);
- Reddit's Math Subreddit;
- Math Bin - a paste tool for LaTeX/mathematics;
- The Grammar According to West - some useful observations and suggestions for writing mathematics;
- Advice to a Young Mathematician - Chapter VIII from The Princeton Companion to Mathematics.