I often need to convert rich text copied from websites or Word/odt documents to Markdown, and sometimes vice versa. Is there an open source Markdown editor that lets me simultaneously paste in and edit a document both as WYSIWYG and Markdown source code? Preferably side by side.

I've actually been using the Reddit redesign's Markdown editor for this, since it lets me switch between a WYSIWYG text box and a Markdown source code editor, but that's obviously not at all a good option.