Duolingo - Duolingo is a free language learning app for iOS, Windows and Android devices. The app makes learning a new language fun by breaking learning into small lessons where you can earn points and move up through the levels. Read more about Duolingo.
Mochi - Write notes and flashcards with Markdown and study them with spaced repetition.
Zorbi - Spaced-Repetition Flashcards - Other tools are built by boomers, Zorbi is built by students.
Yak Tack - remember. words.
Harken - Flashcard app using smart repetition to improve your memory
FlipToMind - Space repetition app to help you remember everything