Org-mode to Markdown converter?

Newer versions of org-mode (>= 8.0) can export to markdown directly: https://orgmode.org/manual/Markdown-Export.html

But you should note that there is no menu to export to markdown by default, and you need to add the following configuration to your .emacs file such that emacs automatically loads markdown export mode: https://stackoverflow.com/questions/22988092/emacs-org-mode-export-markdown

(eval-after-load "org"
  '(require 'ox-md nil t))

It looks like there's still some work to be done, but this project may meet your needs.

So, it turns out you can do this now, with the shiny new markdown exporter

http://orgmode.org/org.html#Markdown-export

If you are looking for a tool to do this outside of Emacs, you should definitely look at Pandoc. In addition to allowing you to convert orgdown files to a multitude of file formats, you can notably also add style to the document at the same time. In other words, just like how we split up HTML and CSS, with pandoc we can keep the design of the document separate from the content.