[Kantara]

I find Human Japanese really interesting and informative. It gives background material on the origin the Japanese language and has a really nice UI. Though its definitely for beginners as it doesn't even touch kanji that I noticed.

Its good for learning the Hiragana and Katakana and leaving nouns, verbs, and the such and you also get exposure to other goodies like counters for objects, animals, etc. Check it out on itunes.