CleverDeck - Learn languages more efficiently with spaced repetition
SmartCards+ - Spaced Repetition Software for iOS. Modern, powerful and easy to use.
ACT Flashcards from Magoosh - Everything you should have learned in high school
FlipToMind - Space repetition app to help you remember everything
Anki - Anki is a program which makes remembering things easy. Because it's a lot more efficient than traditional study methods, you can either greatly decrease your time spent studying, or greatly increase the amount you learn.
Zorbi - Spaced-Repetition Flashcards - Other tools are built by boomers, Zorbi is built by students.