I have this habit of selecting lines from a news/article when I read it and sometimes I double-click a word. I don't know why I do that, but it has become a habit of sorts. Probably, it shows my mentally stressing/bookmarking of some lines and words.
Today when I was reading this article in the NYT, I clicked a particular word and instantly it opened up a reference search "powered by Answers.com".
This I feel is very innovative and very useful for readers. Usually, you will copy and paste this word in your dictionary tool or one of the many only dictionaries, to know the meaning.
Update: This is funny :). When you try to double-click some word on this reference/dictionary page (the screenshot above), it doesn't do anything. It would have been good if they would allow me to search the meaning of other words from this page.