Anki is a flashcard app created by Damien Elmes in 2006. It is open source and widely used by medical students and language learners.
Users make cards or download shared decks, and Anki shows each card again just before they are likely to forget it. Cards answered correctly come back less often, so study time goes where it is needed. Cards can hold images, audio, maths and code. Add-ons extend it with new card types and statistics, and AnkiWeb syncs reviews between devices.
Anki is free on Windows, Mac, Linux and the web, and AnkiDroid is free on Android. The iPhone app is paid, which funds development.