Various math-related databases
For example, databases of various mathematical objects.
The website Catalogue of Mathematical Datasets has a very impressive collection of various databases.
At least some databases are listed here:
- OEIS - On-Line Encyclopedia of Integer Sequences
- π-Base - a database located here: https://topology.pi-base.org/ and https://topology.jdabbs.com/. It could be described as (expanded) online version of Counterexamples in Topology.
- Database of Ring Theory - https://ringtheory.herokuapp.com/ (Here in the Stack Exchange network you can find also a related chatroom.)
Some related posts here on MathOverflow and on Mathematics Stack Exchange: