With the new firefox 2.0, there are two plug-ins which I find really useful. One is to store youtube videos in our PCs (Videodownloader) and the other is from Answers.com, where we get a brief about a particular term on the webpage with just a click
Answers is the add-on that promises to "instantly deliver the information you are looking for". It comes from answers.com, which is a popular online dictionary and Wikipedia syndication site. The Answers add-on works like this:
"Just point at any word, hold the Alt key and click. Upon letting go, an AnswerTip in the form of a pop-up "information bubble" appears on the screen explaining the term."