Other mathematics sites

Video channels, reference works, problem lists and formalization projects elsewhere on the web.

Video

3Blue1Brown. Grant Sanderson's YouTube channel. Animated explanations of linear algebra, calculus, neural networks and individual theorems, built with his own animation library.
Numberphile. Brady Haran's videos, mostly working mathematicians explaining one idea each on brown paper.

Infinity Club. Printable practice sheets for grades 1 to 12, one a night at ten to twenty minutes. Each grade reinforces the school curriculum, adds nearby topics such as primes, binary and imaginary numbers, and shows where the mathematics turns up in music, astronomy and sport.
Numberblocks. The BBC's animated series for ages three to six, in which each number is a character built from that many blocks.
Bedtime Math. A short story and a few questions each day, pitched at three levels from preschool up, for a parent and child to do together.
NRICH. Problems and games from the University of Cambridge for ages three to eighteen, chosen to need thinking more than technique.
Mathigon. Interactive courses for middle and high school, and Polypad, a set of virtual tiles, dice and geometry tools.
Khan Academy. Free video lessons and practice exercises covering the school curriculum from counting to calculus.
Plus. Articles and podcasts from the University of Cambridge on current mathematics, written for older school students.

Quanta Magazine. Reported articles on current research in mathematics, published by the Simons Foundation and editorially independent of it.
What's new. Terence Tao's research blog: expositions of his own papers, lecture notes, and notes on the use of proof assistants and language models in mathematics.

MacTutor History of Mathematics. Biographies of mathematicians and histories of topics, maintained at the University of St Andrews. A source for several entries in the Ledger.
The On-Line Encyclopedia of Integer Sequences. Neil Sloane's database of integer sequences, begun in 1964. Type in the first few terms of a sequence and find where it has appeared before.
nLab. A wiki of mathematics, physics and philosophy written from the point of view of category theory.

MathOverflow. Questions and answers among research mathematicians.
Erdős Problems. Thomas Bloom's catalogue of the problems Paul Erdős posed, each with its current status and references.

Principia Math. Describes itself as “a living, verified map of mathematics.” Members browse and add projects.
Lean community. Home of mathlib, the main library of mathematics formalized in the Lean proof assistant, with documentation and learning material.