“Spaced repetition is a system for permanently remembering something using the minimum number of repetitions necessary. The most popular tool for this is Anki, a free desktop app that enables you to create and review digital flashcards organized by deck.” -Mattan Griffel via http://www.smashingmagazine.com/2014/08/19/mastering-a-programming-language-using-spaced-repetition/.
The great part about Anki however is that it extends the flashcards further.
Select if the question was – easy, good, hard or select if you want to see the card again. Based on which difficulty you select, it will tell Anki how long to wait before showing you the card again.
Another great feature of Anki, the ability to download new decks (flashcard sets). If for instance you want to learn Spanish, or just brush up on it. You can download a Spanish deck, but to go one step further, you can download various Spanish decks uploaded by other users. You can also upload your own decks.
Download the free Anki program at ankisrs.net.
Learn more about Anki at these sites.