SmartCards+ - Spaced Repetition Software for iOS. Modern, powerful and easy to use.
Zorbi - Spaced-Repetition Flashcards - Other tools are built by boomers, Zorbi is built by students.
memordo - Spaced repetition tool to help you retain more knowledge
Harken - Flashcard app using smart repetition to improve your memory
Mochi - Write notes and flashcards with Markdown and study them with spaced repetition.
CleverDeck - Learn languages more efficiently with spaced repetition