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!

Memrise - Learn a new language with games, humorous chatbots and over 30,000 native speaker videos.

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

RemNote - All-in-One Tool For Thinking & Learning

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