Databases

• GroupProps - A Wiki dedicated to groups and their properties

• Liegroups.org -  A database of (a few) Lie groups and their structure theory. May requires you to install their cli software.

• pi-base - Database of topological spaces. Hidden gem!

• Math Counterexamples -  Site dedicated to counterexamples in mathematics.

• Graded rings database - A database of, well, graded rings in algebraic geometry.

• The database of ring theory -  Similar to pi-base, but for ring theory.

• The On-Line Encyclopedia of Integer Sequences - This site helps you match integer sequences against known sequences.

• Jahrbuch Database - A database of old mathematical articles (1868-1942). Unfortunately most links are dead and there is no search field.

Reviews, citations and the likes

• Zentralblatt Now open to the public!

• MathScinet Closed to the public!

• Scopus Citations database.

Miscellaneous:

• Functor Network - A blog platform (run by Chun Deng) specifically designed for mathematical content