Zorbi - Spaced-Repetition Flashcards - Other tools are built by boomers, Zorbi is built by students.
Mochi - Write notes and flashcards with Markdown and study them with spaced repetition.
SmartCards+ - Spaced Repetition Software for iOS. Modern, powerful and easy to use.
Yak Tack - remember. words.
CleverDeck - Learn languages more efficiently with spaced repetition
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.