Anki
apps.ankiweb.net
A free, open-source flashcard program using spaced-repetition algorithms to optimize memory retention for anything you want to learn long-term.
Why it is useful
The most evidence-backed learning tool available — spaced repetition is one of the few educational techniques with strong scientific support for long-term retention. The desktop app and syncing service are free, the shared deck library has hundreds of thousands of community-created decks for languages, medicine, and every academic subject. Used extensively by medical students, language learners, and anyone serious about retention.