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.
Machine Learning Flashcards - 300 digital flashcards
Repetitions - Repetitions is a cross-platform spaced repetition system (SRS), an intelligent flashcard application that makes learning and memorization easy.
Fresh Cards - Flashcards that just work
LearnObit - A tool for your learning, like Anki & Workflowy in one
Flashcards.io - Free offline first flashcards maker