memordo - Spaced repetition tool to help you retain more knowledge
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.
SmartCards+ - Spaced Repetition Software for iOS. Modern, powerful and easy to use.
Quizlet - Quizlet allows you to review and create flashcards for a variety of subjects, such as math and reading.
Harken - Flashcard app using smart repetition to improve your memory
Memrise - Learn a new language with games, humorous chatbots and over 30,000 native speaker videos.