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.
Mochi - Write notes and flashcards with Markdown and study them with spaced repetition.
Memrise - Learn a new language with games, humorous chatbots and over 30,000 native speaker videos.
RemNote - All-in-One Tool For Thinking & Learning
AnkiApp - AnkiApp is a flashcard app for iOS (iPhone + iPad), MacOS, and Android. Get flashcards on any topic. Tracks progress and syncs automatically.