Markdown by DaringFireball - Text-to-HTML conversion tool/syntax for web writers, by John Gruber
NotePlan - Make plans inside an individual markdown note for every day in your calendar. Use it as a journal for your daily tasks and plan todos in advance. For Mac, iPhone and iPad.
Typora - A minimal Markdown reading & writing app.
Markdown to PDF - Super simple way to get your markdown files to PDF
StackEdit - Full-featured, open-source Markdown editor based on PageDown, the Markdown library used by Stack Overflow and the other Stack Exchange sites.
UnMarkDocs - Unleash the Power of Markdown to Write Better Product Documentation