[WahahaNihongo]

You have a lot of dictionaries out there, But I found 2 application(For Iphone only i think)
One is ""Imiwa"(Red icon), and "Japanese"(Red icon)
- The "Japanese" one costs from 5$ to 10$ depends where you live. It has also a good study system for Kanji if you are interested in learning it. also you search by hand drawing like in Chinese messages(Though you need to know the original stroke order for that)
- Imiwa is free, it uses the same database as Japanese, and has the best function when talking with Japanese people by SMS or mail. You just need to copy the message and open the program, it automatically translates word by word with further info on each one.
But hit has no real self practice functions.

So my recommendation is get them both, really helpful!

Hope this helps.