Narkdown Editors (Free)

Multy platforms

https://atom.io/

https://github.com/dvcrn/markright

https://github.com/fbreuer/qute

http://cloose.github.io/CuteMarkEd/

Windows

http://mike-ward.net/markdownedit/

http://markdownpad.com/

MacOS

http://25.io/mou/

http://multimarkdown.com/index.html

Web-based applications