Notepad++ - A free source code editor which supports several programming languages running under the MS Windows environment.
Typora - A minimal Markdown reading & writing app.
Snappify - snappify is a great tool to create and adjust beautiful code snippets easily.
StackEdit - Full-featured, open-source Markdown editor based on PageDown, the Markdown library used by Stack Overflow and the other Stack Exchange sites.
Boostnote - Boostnote is an open-source note-taking app.
MarkText.app - Mark Text is a Markdown editor for Mac, Windows and Linux.