I use markdown extensively on my websites, “Markdown” is two things: (1) a plain text formatting syntax; and (2) a software tool, written in Perl, that converts the plain text formatting to HTML allowing you to build HTML documents in an easily readable form. I tend to use BBEdit a lot but I keep an eye out for markdown editors.
Markdown Pro has been recently updated, this two pane editor allows you to write your document in one pane whilst giving you an instant preview of how the document will look. Markdown Pro now lets you add custom templates to the in built selection of templates. The application was built for Mac OS X so takes advantage of many Mac OS features.