Anki

Anki is a powerful free flashcard program built around spaced repetition — the scientifically proven technique of reviewing information at increasing intervals, right before you're about to forget it, to move knowledge into long-term memory efficiently. Beloved by medical students, language learners and anyone memorizing large volumes of material, Anki is the serious learner's tool: less pretty than Quizlet, but far more powerful and completely free on desktop and Android.

You create decks of cards (or download from thousands of shared decks — the medical and language communities are especially rich), and Anki's algorithm schedules each card's next review based on how well you knew it. Get a card right easily and you won't see it for weeks; struggle with it and it comes back tomorrow. This means you spend your study time only on what you're about to forget, which is dramatically more efficient than rereading notes.

Anki is free on Windows, Mac, Linux and Android; the iOS app (AnkiMobile) is a one-time paid purchase (around $25) that funds development. There's no subscription.

Strengths: the most powerful spaced-repetition engine available, endlessly customizable (add-ons, custom card types, images, audio), a huge free shared-deck library, and it's free forever. Weaknesses: a genuinely steep learning curve, a dated interface, and setup effort that puts off casual users. The payoff for that effort, though, is unmatched retention.

For medical students, serious language learners, and anyone memorizing thousands of facts long-term, Anki is the gold standard — and free.