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.
Zorbi - Spaced-Repetition Flashcards - Other tools are built by boomers, Zorbi is built by students.
Wordd - Quickly look up vocabulary, abbreviations, acronyms, slang
Mochi - Write notes and flashcards with Markdown and study them with spaced repetition.
Easy Ten - Learn any language with ten words a day.
SmartCards+ - Spaced Repetition Software for iOS. Modern, powerful and easy to use.