Haven't tried this, so I can't comment, but some Mac users might find this program useful:
http://www.aorensoftware.com/LanguageAid/
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Overview
Language Aid allows you to simply hover your mouse cursor over any piece of Japanese text and with the click of a button or the tap of a keystroke, have dictionary lookups presented to you in a clean and simple floating window. Features include:
* Customizable lookup trigger to either a mouse or key combination.
* Access to many different dictionaries through Jim Breen's WWWJDIC server.
* Visually scaleable results so that you can see every stroke of that complicated kanji.
* Quick access and dismissal. It is there when you need it but stays out of your way.
Language Aid requires an Internet connection and Mac OS X 10.4 or higher for operation.