I'm a muy grande fan of web app features that - well - just appear, make common sense, and work without any education. Here's one I stumbled onto while sending a link from the New York Times to my sister, via Facebook.
You go to a person's profile [in Facebook]; scroll down to their wall. Then you enter a URL in the wall text box; and the application exposes this widget allowing you to choose which pic [or image] from the URL you want to include in the wall post.
Here's an article from MSDN for a related capability using Popfly:
http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=2725518&SiteID=1
Sounds like fun.
Next is my rave for a word lookup feature on the New York Times site. The implementation isn't as sweet as the FB wall app; but the usefullness is equally high. Basically you double-click any word and a new browser window opens up with the word defined - and this is the cool part - based on the context of the sentence. The site help text is as follows:
Tips
To find reference information about the words used in this article, double-click on any word, phrase or name. A new window will open with a dictionary definition or encyclopedia entry.
I would have preferred a pop-up window; but they might get it right next time :>
con paz,
frank