Have you ever been reading a particularly interesting article in the newspaper and come across some fancy twenty dollar word that the writer slipped in? Well, the New York Times has a built in dictionary feature for every story it publishes online.
Highlight the word, hover over it for and a little question mark appears, prompting the user to "Lookup Word". Clicking the bubble opens a popup window with the definition of the troublesome bit of vocabulary. This is especially useful for scientific or medical focused pieces of journalism.
Thanks to my pal Russell who tweeted this little feature.