Simple, sleek, infinitely useful
Marked 2 must be used with a text editor, but this is a fantastic feature for me. I use Sublime Text 2 (which costs money) but M2 also coordinates well with TextWrangler (which is free). Use the text editor of your choice and easily convert Markdown files (and MultiMarkdown files) into HTML, or DOCX, or PDF files.
There are a few apps out there that do what Marked 2 does, but for more money and greater inconvience. I bought this for the specificity and ease of use. To use you have to open Marked 2 and then open your Markdown editor through the app. Marked 2 is, at its core, a live HTML preview (with options for CSS changes) of the Markdown document you are working on (with other exporting and editing features).
There is also a free 7 day trial on the Marked 2 website, but just remember to delete that if you decide to buy it from the app store. Reading through the documentation on the website is also a great help in finding all of the features of M2, such as the automatic generation of the Table of Contents.
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Marked 2 - Markdown Preview