Tenjin is a free offline Japanese dictionary without ads.
It focuses on displaying relevant data about words, kanji, examples and grammar points in a simple, clear and elegant manner.
Tenjin makes suggestions, searches as you type, and shines at getting the user to the data he wants.
The first time you start the Japanese dictionary, it will download around 20MB (internet needed)...
The free version is great and totally usable.
The unlocked version is exceptional.