Select Math Interactives and Visualizations
These resources have been hand-picked for their easy-to-follow interfaces and general helpfulness. They are in no way meant to be a complete picture of resources—the closest thing to that is the Math Overview Page, aka Awesome Math.
General
Linear Algebra
Interactive Linear Algebra which goes with this course and these resources and this course
Essence of Linear Algebra by 3Blue1Brown
Linear Algebra by Looking Glass Universe
Differential Equations
Differential Equations Explained
Ordinary Differential Equations by blackpenredpen
Calculus
Imaginary Numbers
Imaginary Numbers are Real by Welch Labs
Invitation to Another Dimension
Other Specific Topics
The Chaos Textbook: Mathematics in the Age of the computer
The Wisdom and/or Madness of Crowds