A fun, simple and quick way to learn both the Hiragana and Katakana scripts, StartJapanese! is a flashcard app that allows you to revise in which ever way is easiest for you -- be it your traditional flashcard format or multiple options or even no options, StartJapanese! can do it. StartJapanese! even tests you on the ones you don't know first.
And, not only can you use this app for Japanese letters, you can also use it for any language or subject (from German sentences to the Periodic Table to French or even Spanish) because it allows you to add your own flashcards. Yay!