Anki

Anki is the reference implementation of spaced repetition: cards resurface exactly when you’re about to forget them, driven by the modern FSRS scheduling algorithm. Decks support images, audio, LaTeX, and cloze deletions; AnkiWeb sync is free across desktop and mobile; and the add-on ecosystem plus shared decks (medical school runs on it) is unmatched.

The honest trade: card creation and deck options expose a lot of complexity, and the default styling is spartan — expect an evening of setup and reading before it sings. The iOS app costs money (it funds development; Android’s AnkiDroid is free). It rewards daily discipline and does nothing for you without it.