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.

IMPORTANT: this is not stand-alone software, but requires the desktop client from mnemosyne-proj.org to create cards. Instructions on how to set up the sync server can be found here: http://mnemosyne-proj.org/help/syncing