memordo - Spaced repetition tool to help you retain more knowledge
Yak Tack - remember. words.
SmartCards+ - Spaced Repetition Software for iOS. Modern, powerful and easy to use.
Harken - Flashcard app using smart repetition to improve your memory
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.