As someone who has used the Web since before it had graphics on it, I think that Hyperwords is perhaps the most useful browser addon I've used. It makes selecting text and right-clicking it do infinitely more than "copy" it. With a click you can copy it as plain text as you normally would by clicking "copy", or copy the link the text points to (if it's a link), or search for the text in your choice of search engines, or translate it, or look it up in references....and other stuff I must be forgetting.
Of course which of those items and in what order they appear on the right-click menu is configurable, and you can add new search engines and other resources for it to use so long as they adhere to any sort of standard web search interface, which is just about anything.
I've installed Hyperwords/Liquid Information on many friends, relatives and co-workers computers since it's first pre-public release, and they've almost universally been amazed at how much more useful and easy to use it makes the Web.
In my mind a truly revolutionary addon to web browsers as we have known them since Mosaic. A Must Have!