A little something I found.
RikaiXUL is an extension for
Firefox that pop-up the kanji, kana and english definitions of a word or of a single kanji in a web page.
Just need to right-click anywhere in the web page and click "rikaixul!". Then, just move the mouse on the word you want to check. Very usefull on japanese web page.
You need Firefox 1.0 or later. They're all free btw.
Screenshot
RikaiXUL Extension
Firefox 1.0.4
Enjoy!