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.
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.
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.
AnkiDroid - Memorize anything with AnkiDroid!
Kahoot! - Kahoot! makes it easy to create, play and share fun learning games in minutes—for any subject, in any language, on any device.