This is cool!
It's called Book Burro and it's an extension (there's also a greasemonkey version) that activates when you hover over an item that "looks" like a book. For example, if you're on amazon.com and you go to a book's page, a book burro dialog will appear in the upper left of your browser. If you then hover over the dialog, a price comparsion chart will drop down. Extremely cool!
I found the extension at ecmanaut, a blog I discovered when John at Freshblog referred to it earlier today. Thanks, John.
You design fanatics should be interested in ecmanaut. He likes to play with ecmascript, and now I'm going to have to find out just what that is. This dude sometimes can't resist "another quick hack before going to bed." Woah.
Further info: ecmascript @ Wikipedia. That cleared things up a bit.
Technorati tag: ecmascript | bookburro