I've long been a fan of the 1.x version of MultiMarkdown Composer, and at $5 for the newly released 2.0 version it's difficult to beat. The feature that makes this the best is the Table of Contents view that gets auto-generated from your headings (<h1> to <h6>). It helps so much when navigating long documents. Version 2.0 of this app allows one to rearrange sections of your document! Check out the link to see all the features. PS The app developer is the author of MultiMarkdown itself!