Just came across it and am really liking it. You can leave it open and set it to automatically look up words you copy to your clipboard. The kanji writing breakdown is really good too as you simply hover your mouse over parts of the kanji and it gives you the definition of the kanji. Shows writing order (broken out by word) as well as related words.

Plus, there’s a bunch of other study tools, list making abilities and other good stuff built in.

To top it all off, it’s free.

http://www.tagaini.net/