Minimalist Markdown Editor - This is the simplest and slickest Markdown editor.
Tree - Write documents in tree-like organisation with Markdown
Markdown by DaringFireball - Text-to-HTML conversion tool/syntax for web writers, by John Gruber
TabNotes - Markdown notepad in your new tab
Typora - A minimal Markdown reading & writing app.
Array - "Need a multi-user database application? Code it with HTML/OS.