KanjiBox is an iOS app designed to teach Japanese Kanji and Kana (letters) as well as vocabulary. Moreover, it is organised in such a way as to help you study for the various levels of the Japanese Language Proficiency Test -- the main certification used in Japan to assess a non-native speaker's Japanese language ability.
The app's main way of teaching -- be that kanji or vocab -- is a flashcard game that shows you four possible answers on the upper half of the screen and a hint on the bottom half. If you choose the correct answer, that hint will only come up rarely as you continue to play. But if you get it wrong, the hint will come up a lot more often. Also, you are able to customise the hints -- to show the phonetic reading, the English meaning, a sentence with a missing letter, or any combination of these -- in the settings menu to help you better focus in on your weak points.
In addition to the flashcard drill mode, there is a quiz mode that adds a timer and scores you not only on if you answer correctly but also on how long it takes you. The faster you answer (correctly), the more points you score. Every few questions, the type of hint also changes, requiring you to know the information in every possible permutation. While ostensibly the same game as the normal drill mode, the timed score and random hint types make it a much greater challenge.
In addition to the flashcard drill and quiz modes available in the $3.99 core program, there are also a few useful DLC options. There are several grammar packs -- so you can study grammar in addition to the kanji and vocab (for $0.99 a set). And for those who wish to practice writing kanji, you can purchase an additional mode (also for $0.99) that teaches that as well.
As it's been several years since I last studied Japanese, I have found this app to be a great review. Words, grammar, and kanji that I don't tend to use in my daily life have come flooding back to me. But the thing I love most about the app is that it allows me to study in short bursts whenever I have a free moment -- like when walking or on the train. If you are looking for a good, cheap way to bolster your Japanese ability, KanjiBox is a great choice.