Nihongo is a feature-rich Japanese dictionary / flashcard app that addresses the shortcomings of many other popular dictionary & flashcard apps. It's quickly becoming my go-to dictionary / flashcard app.
* Free (with paid add-ons), so no risk in trying it out.
* Responsive and functional dictionary search that sorts based on how common the word is. I love how it gives you the "Best Matches" to help you determine the most natural Japanese-sounding choice.
* The flashcard function is far beyond the flashcard feature of other dictionary apps I've tried. It takes the tedium out of making them yourself and tests you in ways usually found only in dedicated flashcard apps.
* Retains your search history, which it can then turn into flashcards. Very cool!
* You can even add your own notes and mnemonics to your flashcards--a sometimes hard-to-find feature I absolutely demand.
* I wish it had "real" Japanese handwriting recognition (like Midori and Renzo's Japanese dictionary) instead of relying on the iPhone's Chinese input method.
* I wish it had kanji stroke order animations.