In roughly increasing order of mathematical advancement:
Paul's Online Notes,
a useful Calculus (and Algebra and Differential Equations) resource.
(After reading the terms, find a course from the top menus.)
- Piazza, my favorite Q&A platform for classes.
- Mathematics: A Very Short Introduction by Timothy Gowers. My favorite short introduction to advanced mathematics for an undergraduate.
The Hundred Greatest Theorems
according to Nathan Kahl.
- More links from Anton Lukyanenko.
- SageMath, free mathematical software system (including a cloud version).
- I use TikZ for drawing figures in LaTeX.
- MathOverflow, a nice site for graduate-level questions.
- Nurikabe: Daily moderate or harder puzzles. (I would love to see a Nurikabe puzzle that has a unique solution with the additional constraint that each possible Tetris piece is an island.)
- NetWalk daily puzzles.
- Lots of (easy) applets: Ed Friedlander's site
- Even more variants, plus some analysis: ChessVariants.org (I am an editor of this site.)
- 3+ dimensional analysis: CV Wikidot
- A partial list of my favorites:
- My own 4D game, and some thoughts: Chess-Basics
- A mashing together of chess variant and deck-building game: For the Crown