Spaced repetition is a revision technique that minimises the number of exposures to a piece of subject matter required to commit it to longterm memory. This is achieved by altering the frequency in which items of revision material are shown to the user based on how easily it was recalled last time; the content is shown to the user right before he or she is due to forget it - and not before, avoiding wasting time on premature revision. Anki is a free and popular tool available on Windows, Mac and mobile devices that allows you to synchronise your progress between all your devices via Ankiweb.