Actually, this is only a must-have extension for people who struggle with reading Kanji. I assume that is quite a number of you reading this, though. The Firefox extension rikaichan does exactly what we would often like to do when trying to read a Japanese text: by simply hovering over a kanji, the English translation pops up.
As you can see, it shows both the individual Kanji as well as the whole word composed of several Kanji. Installation is as easy as clicking on the download link for the add-on itself and then the Japanese-English dictionary file. Hitting the shift key switches from the translation to a detailed view of the Kanji, including all the radicals. The FAQ has lots of other information. If you have not used Firefox yet, this add-on alone is worth giving this browser a try.