LingQ

Created by Steve Kaufmann, a Canadian polyglot who speaks 20+ languages, LingQ is built around the idea that massive input — reading and listening to content you actually care about — is the fastest path to fluency. You import articles, podcasts, YouTube videos, and ebooks, then click unknown words to save them as "LingQs" with translations. The system tracks your known word count as a progress metric. It's not gamified or pretty, but the input-based approach has a cult following among serious language learners.