Maybe this is an old feature but I just noticed it today. When you have text selected in any Cocoa application, you can go to the Services menu and select "Summarize." This opens a new app called Summarize Services which creates a summary of the text and let's you use a slider to determine the summary's length (from 1 to 100% of the original). I can't see how anyone would find this useful, but it's certainly neat-o.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020829054124931