Below are links, grouped into categories, to sites that I find useful and/or interesting.

#### mathematics

• 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.