Anki

Anki is the undisputed efficiency champion for vocabulary retention, using open-source spaced-repetition algorithms that boost long-term recall by 50% over traditional study methods. Its community-created decks cover virtually every language, with over 10,000 pre-built card sets available. This tool outperforms #5 Busuu in raw memorization power, though it lacks the interactive feedback that makes Busuu better for speaking practice.