While reading the review for Black Snake Moan at the NY Times, I came across a word I didn't know: cornpone. So, I double-clicked it so I could copy and paste it into the google search window. Before I could reach the letter C, a window had popped up with the definition. This is the perfect execution of a feature I've always wanted. I'm surprised it's taken this long to surface.

If you want to try it yourself, you should give it a shot with Firefox first. It may work in IE, but it definitely doesn't work in Safari. Also, it only seems to work on article pages. Here's a video of the pop-up in action (you can't quite the see the double-clicking and highlighting thanks to compression by Snapz Pro and Youtube):