this app is the BEST English-Japanese dictionary out there. Actually it's the best cross-language dictionary I've used period. Although I admit, I mostly look at free ones, often paid ones I find are junk.
this app makes it so easy to look up terms in romaji, or if you've installed Japanese keyboards on your phone/ipad you can look up by hiragana or kanji directly. If you install the one Chinese keyboard that lets you draw the characters it's even better.
Once you've found a word, you can see example sentences, when they are available. You can also see the breakdown of the kanji in the word, and continue to see what that particular kanji means. The kanji definitions have all the readings, radicals, and brush strokes.
this app lets you have multiple lists. I save lists for fun phrases, cool words, kanji I need to study, and (when that was a concern) words for the different JLPT levels.
...I use it a lot in the car to look up a Japanese lyric I just heard. Or a random word I suddenly realize I don't know the Japanese word for but I should.
If you are studying Japanese, or even if you just like anime, it's a no-brainer to install this app on your phone.