Anki is a powerful, open-source flashcard program renowned for its effective spaced repetition system (SRS). It isn't a language course itself, but a highly effective tool for memorizing vocabulary, phrases, or grammar rules that you input yourself or download from shared user-created decks. Its high degree of customization allows for rich cards with audio, images, and cloze deletions. While its interface can be unintuitive for new users, its effectiveness for long-term retention is largely considered unparalleled for dedicated learners.

Clozemaster is a language learning game focused on building vocabulary through mass exposure to sentences. The core mechanic involves filling in a missing word (a "cloze") in a sentence. It uses a spaced repetition system to re-test words and features a heavily gamified interface with points and leaderboards. Sourced from a large corpus of translated sentences, it's an excellent tool for intermediate learners to move beyond basic vocabulary and see words used in a wide variety of contexts, which can implicitly reinforce grammar.