Plan your route and find out train and bus fares at the same time. Very useful for planning long journeys in Japan or just checking when the last train home is.
Type or dictate a sentence into this app and Google Translate will feed it back to you in Japanese. This is immensely handy for travelers as you can select text to be displayed in a kind of 'flashcard' mode (only the text) so it can be shown to others when asking locals for help. The app can also speak the Japanese translation out loud for you. Looks like I just wasted three years at university studying Japanese! Be a little wary of the translations though; the translating is performed by a bot which can sometimes miss the context or use an outdated word so you may not always get an accurate translation.
Where Google Translate fails, Kotoba! will usually make up for it. More modern translations are available as well as a very useful search-for-kanji-by-radical feature.
Travelling but stuck with nothing to do? You'll be surprised how many of these are hidden around Japan, even in the country side. This app encourages you to journey to different and interesting places based on user-generated content.
Handheld puri-kura machine. Make your photos kawaii!
6. hello Japan
One quick shake brings a wealth of inspiration to your fingertips: hundreds of articles, experiences and tips on Japan written entirely by locals in every area of the country.
I'm sure there is much more that should be added to this list, especially with regards to Japanese language learning. What is your top iPhone app?