Anki

The ugly, unintuitive flashcard app that outperforms everything else. Anki's spaced repetition algorithm is based on decades of memory research, and its open-source community has built shared decks for every language imaginable. Medical students, polyglots, and competitive memorizers all converge on the same tool. It looks like software from 2003 because it is — and it works better than anything released since. Free on desktop, $25 on iOS, free on Android.