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.

AnkiDroid - Memorize anything with AnkiDroid!

Duolingo - Duolingo is a free language learning app for iOS, Windows and Android devices. The app makes learning a new language fun by breaking learning into small lessons where you can earn points and move up through the levels. Read more about Duolingo.

Quizlet - Quizlet allows you to review and create flashcards for a variety of subjects, such as math and reading.

Kartka - Kartka is a flash card learning tool with real-time sync, spaced repetition algorithm, and a modern and friendly interface.

Mochi - Write notes and flashcards with Markdown and study them with spaced repetition.