I think I may collapse from sheer wonder and joy. This is the way the web is supposed to work. AND IT ALREADY DOES AND I DIDN'T KNOW IT.
I just added the
Hyperwords extension to my Firefox browser and Oh. My. Sweet. God. Now I can highlight a word/phrase and a menu appears so I can pop up the dictionary.com definition of it or the wikipedia entry for it or a translation or a search. I can highlight a number and pop up a conversion for temperature or currency. And so many other things -- I only have a few of the options turned on; just the ones I'm likely to use right now, but you can customize it for all kinds of things.
And it's sexy-looking and easy as hell (I'm using the icon option instead of the right-click option, but you can use it either way), and the dictionary.com or wikipedia page it pops up for you with your word pre-entered is also sexy -- and you have options there, too; you can click in the upper left corner of that popped-up wikipedia entry and it will move to appear in a new tab, instead of a pop-up window, and oh holy cow, I'm in freaking love.