Typora - A minimal Markdown reading & writing app.
MacDown - MacDown is an open source Markdown editor for OS X, released under the MIT License.
Markdown by DaringFireball - Text-to-HTML conversion tool/syntax for web writers, by John Gruber
iA Writer - Minimal Design, Maximum Focus
Gingko - Write and organize your ideas in Gingko tree documents.
MWeb - Pro Markdown writing, note taking and static blog generator