Love this app!

Finally, a document processor that I love! It lets you use any text editor you want, like BBEdit, Vim, whatever, and write in the MarkDown markup. Then, when you do a save in your text editor, the Marked 2 window shows you what your document will look like. Very easy to use, very easy learning curve, very nice looking app. You can export your document to one of many different formats like PDF, RTF, DOCX, HTML, etc. I am using it to document my code using the GitHub flavor of MarkDown. Great app! Highly recommended.

bitsmith about Marked 2 - Markdown Preview