OmniOutliner Markdown Plugin
I have been experimenting with an XSLT that transforms OPML into Markdown text. It works fairly well, but there are issues that arise from the fact that OPML is poorly designed. In addition, it would be nice to be able to export straight to Markdown from OmniOutliner, rather than saving an OPML, and then running xsltproc to convert.
So I created an Export plugin for OmniOutliner that allows you to save your outline as Markdown formatted text.
See MultiMarkdown and OmniOutliner for more information.