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