Brett Terpstra has released Marked 1.3.1 for Mac OS X (10.6 or higher), an update of his viewer for Markdown and MultiMarkdown files. It costs US$3.99 and is available worldwide through the Mac App Store in the Productivity category.
Version 1.3 allowed the use multiple custom styles for the preview, preview improvements (smooth scrolling, automatic Table of Contents, MathJax) and the ability to open and text file regardless of its extension. Version 1.3.1 handles larger files, provides more precise error messages, adds more keyboard shortcuts and preserves metadata when rendering the preview. As of version 1.3, Marked can also watch the active CSS file for changes in addition to the Markdown or HTML file and update the Preview when style changes are detected.
Marked turns any text editor into a Markdown editor, allowing you to edit when you like, where you like. It can track changes in the file and update the preview automatically on every save. Marked allows for HTML export, printing, save to PDF, copying HTML source directly to the clipboard, document statistics and more.