Since it’s introduction, MultiMarkdown Composer (MMDC for short) has steadily added new features and improved MMD handling and it’s a pretty great general purpose editor now.
MMDC continues to be one of the most cutting edge text editors for the Mac. For example, this latest release includes Mavericks tag support. Not only will
it offer to convert OpenMeta tags to Mavericks tags (As the developer notes in the comments, this feature is not available yet. Sorry. I misread the release notes.) but it will auto-suggest Mavericks tags.
There’s also a discussion board where you can grab more themes for MMDC.
I’ll selfishly add that it is the best text editor for working with CriticMarkup since it supports preview and built-in functions for marking text with comments.
Checkout this tremendous list of features on the developer page.