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.
LearnItFast - LearnItFast solves the problem of forgetting of online courses and study notes.
Quizlet - Quizlet allows you to review and create flashcards for a variety of subjects, such as math and reading.
Memrise - Learn a new language with games, humorous chatbots and over 30,000 native speaker videos.
Mochi - Write notes and flashcards with Markdown and study them with spaced repetition.
Brainscape - Find, create, and study SMART FLASHCARDS on any device. DOUBLE your learning speed using the most effective study system on the planet. Keep all your content in sync across Brainscape's website and your Android devices.