Other Sudoku sites

My Sudoku is not the only place to learn and play. Here are well-known sites, by what they are good for.

Learn the techniques

SudokuWiki.orgsudokuwiki.org
Andrew Stuart's site: a solver that explains every step, and the strategies, from the simplest to the most advanced, documented with examples (in English).
HoDoKuhodoku.sourceforge.net
A free solver, generator and trainer, with a detailed guide to solving techniques (in English and German).

Cracking the Crypticyoutube.com
A YouTube channel where expert solvers tackle Sudoku and its variants, thinking out loud (in English).
Sven's SudokuPadsudokupad.app
A web app to play Sudoku and its variants, the one Cracking the Cryptic uses.
Web Sudokuwebsudoku.com
Free puzzles to play online, at four levels (in English).

Nikolinikoli.co.jp
The Japanese publisher that gave Sudoku its name, and still makes its puzzles by hand.
World Puzzle Federationworldpuzzle.org
The international federation behind the World Sudoku Championship, held nearly every year since 2006.
Logic Masters Deutschlandlogic-masters.de
A German club for logic puzzles, which runs Sudoku championships (in German).
Wikipedia: Sudokuen.wikipedia.org
The puzzle's history, rules and variants.
Wikipedia: Mathematics of Sudokuen.wikipedia.org
How many grids there are, the fewest clues a puzzle can have, and more.

These sites have no connection with My Sudoku.