Some links to stuff that I find interesting.

Academic resources
Keith Conrad's blurbs Hundreds of short expository notes across algebra, number theory and analysis.
The Rising Sea Daniel Murfet's course notes and videos on category theory, algebra and logic.
Dragan Miličić Lecture notes on Lie groups, representation theory and real analysis.
Miličić on derived categories Introductory notes on derived categories (PDF).
Pete L. Clark's expositions Expository notes on algebra, number theory and general mathematics.
The Catsters Simon Willerton and Eugenia Cheng's category-theory video series.
Blogs & websites
Byrne's Euclid Oliver Byrne's colour edition of Euclid's Elements, rebuilt for the web.
math3ma Tai-Danae Bradley's expository posts on category theory.
Quanta on homology Popular-level introduction to how homology detects the shape of a space.
What's new Terence Tao's research blog and lecture notes.
Gowers on Zorn's lemma Timothy Gowers on how to actually use Zorn's lemma.
math.stackexchange #684007 An answer worth keeping around.
The n-Category Café Group blog on category theory, physics and the philosophy of mathematics.
Club de Matemáticas, Yachay Tech Student mathematics club at Yachay Tech.
More stuff
Gilles Castel Fast LaTeX note-taking in Vim and figure drawing with Inkscape.
Luke Smith Linux, terminal tooling and a minimal computing setup.