Writer Pro for Mac and iOS
This app is designed around the four basic stages of any writer’s workflow – note-taking, writing, editing, and reading – each of which is represented by a different typeface and color. As you work on a given document, you can toggle it through the various “stages” as needed.
The real killer feature, though, is what they’re calling “Syntax Control”. When enabled, you can choose a specific part of speech (adjectives, nouns, verbs, adverbs, etc) and Writer Pro will fade out everything but the words that fit that description. This makes it super easy to locate repetitive words and weak phrasing in your writing.
Writer Pro is available for the Mac and as a universal iOS app. Each version is $20. I recommend reading their blog post that covers the decisions behind their pricing and thus-far lack of Dropbox integration.