"Japanese" by codefromtokyo.
This is assuming your studies aren't from scratch.
It's a dictionary prog using the JMdict/EDICT/KANJIDIC2 databases which provides great translations, examples, and it uses an awesome touch interface to get more info on specific kanji, or see different compounds that use the same kanji.
It also supports the building of vocab lists to study in flash cards a la Letiner method (w/ stacks), and integrated JLPT vocab and kanji lists broken down by level.
It costs $9.99 in the US store, but it's totally worth it.