Anki

Anki is a flashcard program built around spaced repetition. Each time a card comes up you say how easily you recalled it, and the program decides when to show it next: intervals stretch out for material you know and stay short for material you keep getting wrong. Review work is spread across many short sessions instead of piling up before a deadline.

Cards are not typed in directly. You enter notes made of named fields, and card templates turn each note into one or more cards, so a single vocabulary note can produce both a recognition card and a production card. Fields hold formatted text, images, audio and LaTeX, and cloze deletion hides part of a sentence rather than splitting it into separate notes. Decks nest inside one another, cards take tags, and filtered decks gather cards by search for a focused session.

The desktop program keeps everything in a local collection file, synchronises with the AnkiWeb service and its mobile clients, and imports decks shared by other people. Add-ons written in Python extend it, and the scheduling parameters can be adjusted per deck.