Chess.com - Lesson Guides
Lichess.com - Lesson Guides
Chessable - Interactive Courses
ChessPuzzle.net - Puzzles from Tornament Games
ChessStrategyOnline.com - Tutorials
Chesstempo.com - Rated Puzzles
Chess Kid - Safe, Fun Learning Environment
A kids Guide to Playing Chess
Lichess Kids - Subsite of Lichess
FIDE - World Chess Federation, rules and ratings.
The Week in Chess (TWIC) - Weekly downloadable PGNs and tournament coverage.
365 Chess - Massive searchable game archive.