Evernote

Evernote is a note-taking application that keeps notebooks across platforms and uses OCR to index images.

It evolved into a web-backed, cross-platform service (more compelling than Yojimbo and similar scrapbook apps).

Using ScriptingBridge, a bit of Python, and html2text, you can draft in the WYSIWYG editor and export as Markdown. There are quirks with spacing and lists, but it works:

from Foundation import *
from ScriptingBridge import *
import html2text

e = SBApplication.applicationWithBundleIdentifier_("com.Evernote.Evernote")
buffer = e.selection()[0].HTMLContent()
# I prefer reference-style links
html2text.INLINE_LINKS = False
print html2text.html2text(buffer)