On yesterday's MacBreak Weekly show on the TWiT network, Andy Ihnatko announced Hemmingway as his app of the week. BRILLIANT! I'll never write again without it.
Hemingway is a fantastic app for writing that helps you write more clearly. It offers a number of features beyond your standard word processor, including highlighting long and complex sentences, giving you a readability level indicator, highlights adverbs, and gives you hints on simplifying your writing and avoiding passive voice.