Open Source:
MacDown [1] is a pretty decent MIT licensed editor. It's probably one of most well known open source Markdown editors for OS X.
Haroopad [2] is GPLv3 licensed and does pretty much everything that MacDown can. It is also cross platform (Windows, Linux, and OS X) which is an advantage to some.
Both offer the ability to change their look via themes and support various Markdown flavors.
Paid:
Byword [3] and iA Writer [4] are both very nice minimalist editors. They both have 'focus' modes which dim all the lines and/or paragraphs in the document except the one you are on to help you concentrate while writing, etc.
[1] http://macdown.uranusjr.com/
[2] http://pad.haroopress.com/
[3] http://bywordapp.com/
[4] https://ia.net/writer/ios/