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