Japanese.app for iPhone: 最高
Although there are many Japanese dictionary/tool apps on iPhone, including at least one fairly decent free one, the $20 Japanese.app is not only the best fucking Japanese reference app on iPhone, but also the best Japanese reference app I have ever used on any platform, including all the weird proprietary platforms developed solely for providing an electronic Japanese reference tool.
I cannot wait to pony up some more ¥ for the Mac version, which they say is planned.
It kicks too much ass for me to even describe; I'd get tired. Just buy it if you need the best Japanese reference tool available.