Anki
apps.ankiweb.net
Schedules flashcard reviews for the day before you would have forgotten them
Free account
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apps.ankiweb.net
Schedules flashcard reviews for the day before you would have forgotten them
duolingo.com
Turns a language into five-minute daily drills you will actually keep doing
exercism.org
Practice exercises in 80-odd languages with free feedback from a human mentor
freecodecamp.org
Teaches web development by making you write code in the browser from lesson one
kaggle.com
Community-uploaded datasets with notebooks showing how people used them
khanacademy.org
Explains school maths and science one short video and one exercise at a time
ocw.mit.edu
Publishes the real lecture notes, problem sets and exams from MIT courses
monkeytype.com
A typing test you can tune to the exact words, punctuation and length you want
musictheory.net
Free lessons and endless drills for notation, intervals, chords and ear training
openstax.org
Peer-reviewed university textbooks you can read online or download as PDFs
rewordify.com
Rewrites difficult English into simpler words, keeping the meaning
Where these come from
A real job usually crosses two or three categories, which is why starting from the task is often faster than starting from the shelf.
Questions
Practice-first sites beat lecture-first ones for anything you will do with your hands. Pick one with a visible syllabus and a way to check your work, and finish something small in the first session — completion rate is the only metric that matters.
Yes — 5 of the 11 sites listed here work with no account at all. They are marked "No sign-up" and you can spot them by the green rule down the left edge of the card.
1 of these do the work inside your browser and never upload the file — those are the ones to use for anything with personal data in it. The rest send your input to their servers, which is fine for public material.