Hyperwords lets you interact with the text on the webpages. For example, if you see a word you want to look into it, you probably will highlight the word, then copy it, then open up a new tab, go to wikipedia or google, then paste the word and click search. It’s quite a long process isn’t it? With Hyperwords all you need to do is to highlight the word, then right click and find what you want to do with the word, for example google this word.

There are a lot more features. You can watch a demo at:
http://www.hyperwords.net/demo.html

The firefox extension of this is located at:
https://addons.mozilla.org/en-US/firefox/addon/1941