Typora - A minimal Markdown reading & writing app.
MacDown - MacDown is an open source Markdown editor for OS X, released under the MIT License.
StackEdit - Full-featured, open-source Markdown editor based on PageDown, the Markdown library used by Stack Overflow and the other Stack Exchange sites.
iA Writer - Minimal Design, Maximum Focus
MarkText.app - Mark Text is a Markdown editor for Mac, Windows and Linux.
MWeb - Pro Markdown writing, note taking and static blog generator