Mnemosyne is a feature-rich flashcard memorization tool. The Mnemosyne software resembles a traditional flash-card program to help you memorise question/answer pairs, but with an important twist: it uses a sophisticated algorithm to schedule the best time for a card to come up for review. Difficult cards that you tend to forget quickly will be scheduled more often, while Mnemosyne won't waste your time on things you remember well. There are dozens of card sets available on the app homepage.