The Hemingway app is what Word could be if it had a more informative, tunable interface. You type in some text, and you get feedback on things like run-on sentences, word misuse, much more. The interface allows you to turn on and off checks for hard to read sentences, adverb overuse, passive voice misuse, and more.
This is the tip of the iceberg as to what might be done with an interface like this. Helpful to professional writers, but could be an incredible boon to writing students. I would love to see technology like this built into Safari.
UPDATE: Reader Jim “The Beard” Dalrymple pointed me to this post which runs Hemingway’s actual writing through the Hemingway app. Pretty funny.