Atlas-like websites on specific areas of mathematics

A classic: The On-Line Encyclopedia of Integer Sequences (OEIS).

• Atlas of Lie Groups and Representations: info on lie groups, there is a nice article on the history of this atlas

• Atlas of Finite Group Representations: info on finite groups actually the succesor of a book (already mentioned by OP but added it anyway to keep it symmetric with the first point)

• Complexity zoo:list of complexity classes and what is know about them

• Quantum algorithm zoo: list of quantum algorithms

• Knot Atlas: list of knots

• Encyclopedia of triangles center: list of triangle centers

• Bilbao chrystallographic server: list of space groups and info although more physics oriented

• Googology wiki: Info on googology

• Digital Library of Mathematical Functions: a spiritual successor to a book too and there is also a nice article on the history

Forking and dividing is a map of the model theory universe, visually classifying some 63 first-order theories.