Anki

Anki is a flashcard program built on spaced repetition: each card comes back after a gap that grows every time you remember it and shrinks when you forget. That way you spend review time on what you are about to lose, not on what you already know.

You can make your own cards or download shared decks for languages, medicine, law and many other subjects. Cards can hold images, audio and formatting, and add-ons extend what the desktop app can do.

The desktop app and the Android app are free, the iPhone app is paid, and AnkiWeb syncs between them. It takes some setup, and people who stick with it often use it for years for languages and exams. Shared decks save time on common subjects.