Duolingo

The most downloaded education app in the world with 100+ million monthly active users learning 40+ languages. Duolingo's gamification (streaks, hearts, leaderboards) exploits the same psychology that makes social media addictive — except it teaches you Spanish. The free tier is comprehensive: all lessons, all languages, with ads. Super Duolingo ($7/month) removes ads and adds unlimited hearts. Studies show Duolingo is as effective as university courses for reading and listening comprehension (less effective for speaking). The green owl's passive-aggressive streak notifications have become a cultural meme. Duolingo went public in 2021 at a $6.5 billion valuation.