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!

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

LearnObit - A tool for your learning, like Anki & Workflowy in one

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

Repetitions - Repetitions is a cross-platform spaced repetition system (SRS), an intelligent flashcard application that makes learning and memorization easy.