The New York Times just introduced silently a very neat feature. Double-clicking any word will open a page which shows by default the definition of that word (Answer.com/Reference). At the top of the page, there is a search box for NYT archives, NYT blogs, Google, etc. It does not work everywhere but it does on this article: Fuel Without the Fossil which is part of the Energy Challenge series.

It is simple, elegant, non-obtrusive and certainly useful.