The iDevice is an excellent platform for language acquisition apps. Languages which use especially benefit from the svelte touch screen and multimedia slant. iKana touch is an exquisite app for students of Japanese to learn both official syllabaries: hiragana and katakana.
Nihon IR, maker of PC Dr. Momo, along with the Japan Times, have an ace up their sleeves; 出る順 is an excellent Japanese-English study tool as well as a great reference for learners of Japanese. It is a set of true language study implements that are geared toward the Japanese learner of English, but also serves as an excellent Japanese learning reference.
To become acquainted with 出る順, you have to be willing to experiment if your Japanese is stuck at difficult kanji. However, as it is a reference for Japanese learners of English, the main bulk of users should be comfortable with all aspects of text, layout and differing modes of study. (By the way, this review was written on QuickOffice for the iPhone!)