Org-mode to Markdown converter?

I have an org-mode file that is mostly just an outline, and want to convert it to Markdown. But it is a long and big file, so is there a piece of software that can do this for me? I use Mac OS X, but am willing to do this in another OS if needed.

Thank you.

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