Links
Mathematics
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 graduatelevel questions.
Puzzles
 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.
Chess Variants
 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: ChessBasics
 A mashing together of chess variant and deckbuilding game: For the Crown