I've seen this Firefox extension pop up in a few places this morning, so when I got a chance, I installed it so I could take a look. Amazing.
You know how you're reading a story in your browser, you get interested in a particular phrase, and you get distracted as you Google for it, check Wikipedia about it, and so forth? Juice does all that research for you in a side pane. You highlight the phrase, drag it slightly to the right, and the rest of the work is done automatically. It might be enough to make me stick with Firefox as my primary browser even after Chrome for OSX comes out.