Another excellent innovation from the chaps at mozdev.org - the Moji extension for Firefox and Thunderbird. Moji is an integrated sidebar-dictionary aimed at helping people read Japanese (and Chinese) on the web. Simply select a single kanji or a whole word from whatever Japanese text you are reading and click on the respective button in the Moji sidebar to display the kanji reading and English meaning. The killer aspect of Moji is that any terms you look up stay in the sidebar until you erase them, so you can use Moji to read an article in Japanese, accumulate a list of all the kanji/words you couldn't read and then export that list as text or html. This is what differentiates it from say, rikaichan, which gives you the appropriate reading on the fly without letting you make a list. I'd use Moji for those times when you want to intensively study a particular article and have a record of it and use rikaichan for troublesome kanji encountered when scanning the news or reading for pleasure (so as to avoid disrupting the flow of your reading).
Moji is based on EDICT and only works from Japanese/Chinese and has a whole bunch of dictionaries available for download including French, German and Russian. You can download Moji from the mozdev.org website.