Google Keep - Capture notes, share them with others, and access them from your computer, phone or tablet. Free with a Google account.
Minimalist Markdown Editor - This is the simplest and slickest Markdown editor.
GitJournal - Manage your Notes from any Git Repo.
Typora - A minimal Markdown reading & writing app.
Quillnote - Take beautiful markdown notes and stay organized with task lists.
Markdown by DaringFireball - Text-to-HTML conversion tool/syntax for web writers, by John Gruber