Anki

Anki is the most widely used cross-platform spaced repetition system built on the same SM-2 algorithm that powers CleverDeck. It offers completely free desktop and Android apps plus a paid iOS app, massive user-shared language decks, and powerful add-ons. Unlike CleverDeck's curated 3000-word sets, Anki relies on community content and requires more initial setup, but it works on every device and has no subscription cost for core use.