Anki

Anki is a flashcard application built on spaced repetition. A card has a front and a back, and after each review the learner rates how difficult the recall was. That rating determines when the card is shown again, so material that is easy is seen rarely and material that is hard is seen often. The scheduling algorithm is the product. Decks are user-created, and the shared deck library means a learner can start from someone else's deck for a standard subject. Because the algorithm is open and the file format is documented, decks can be generated programmatically from other data. The interface is functional rather than polished, and creating good cards is work: a poorly written card produces poor recall no matter how good the schedule is. The application is free on desktop and Android, with a paid iOS client that funds development, and synchronisation runs through a free account.