A list of (not necessarily) lesser-known maths resources, maybe recommended, maybe not, in no particular order. Feel free to procrastinate from more important work by exploring the links as I have done through the years, which led to the failure I am now.
Notes / Courseware / Wikis
MAT327 Topology - Ivan Khatchatourian, Toronto
Cambridge Lecture Notes - Zhiyuan Bai, Yale
Cambridge Notes - Dexter Chua, Harvard
Harvard MATH 55A - Denis Auroux
Harvard MATH 55B - Denis Auroux
Books
Linearity, Symmetry, and Prediction in the Hydrogen Atom - Stephanie Frank Singer
Second Year Calculus: From Celestial Mechanics to Special Relativity - David M. Bressoud
[Fundamental University Physics - Alonso, Finn]
Lecture Series
Statistics 110: Probability - Harvard
Multivariable Calculus and Linear Algebra - Theodore Shifrin, UGA
Probability Theory, I, II, III - Todd Kemp, UCSD
Outstanding YouTube Channels
VisualMath - Daniel Tubbenhauer, Sydney
Michael Penn, Randolph College
MathMajor - Michael Penn, Randolph College
Richard E. Borcherds, Berkeley
YouTube Channels
ThatMathThing - Joel A. Rosenfeld, South Florida
Institut des Hautes Études Scientifiques
International Centre for Theoretical Physics Mathematics Section
Outstanding Videos
Percolation: a Mathematical Phase Transition
Videos
3Blue1Brown angrily lashes out at you for being bad at math (POV)
Blogs
Journey In Mathematics - Qi Zhu, Bonn
Frederick Tsz-Ho Fong, HKUST (blog articles in Cantonese)
數學教授的日常生活 - Shingyu Leung, HKUST (in Chinese)
This Week’s Finds - John Baez, UCR
Not Even Wrong - Peter Woit, Columbia
線代啟示錄 (in Chinese)
Lists / Guides
Algebraic Geometry from High School
Highest voted questions on MathOverflow
Questions tagged [big-list] on MathOverflow
Highest voted questions ono Math Stack Exchange
Questions tagged [big-list] on Math Stack Exchange
How to Become a Pure Mathematician (or Statistician)
Tools
Famous People
I put them here partly to simp, and partly to recommend their notes, which I have not read so my recommendation is useless.
Extra Education
Including summer schools, online schools, etc.
Collection of Programs from MAA
List of Opportunities from AMS
Budapest Semesters in Mathematics
Fields Undergraduate Summer Research Program
PCMI Undergraduate Summer School