Anki (baseline reference)
The free, open-source category-defining spaced-repetition tool with the world's largest shared-deck library.
Best for
Users who enjoy deck-authoring as part of learning, and serious medical / language students who benefit from AnkiWeb's mature community decks.
Weakness
Time cost to author cards is high. Mobile UX on iOS is paid (~$25 one-time). No AI generation. Steep learning curve.