Markdown Editors

Markdown text format (variation of Wiki) is likely to have a bright future not only for open source projects (GitHub popularized the format for readme files) but for general purpose text as well.
Tracking changes line by line is much better with markdown than with typical text formats
like .DOC, .HTML etc... Generating web pages and dynamic styling is very easy.

Visual Studio Code - Code Editing. Redefined
Open source and free, for all major OSs; My preferred editor

Markdown editing with Visual Studio Code

Just save files as .md and use Ctrl-Shift-V for preview;
use split view for continuous preview

Atom
"A hackable text editorfor the 21st Century"
open source and free, for all major OSs
GitHub - atom/markdown-preview: Markdown preview in Atom

Markdown Monster - A better Markdown Editor for Windows
Windows "native" WPF, open source but not free app for editing Markdown files.

GitHub - RickStrahl/MarkdownMonster: An extensible Markdown Editor, Viewer and Weblog Publisher for Windows

Getting Started with Markdown Monster - YouTube

interview with the author and links from:
Markdown Monster with Rick Strahl @ .NET Rocks! vNext

Daring Fireball: Markdown

CommonMark
standardization of the syntax, with changed name as response to objections by Markdown creator

"A strongly defined, highly compatible specification of Markdown
What is Markdown?
It’s a plain text format for writing structured documents, based on formatting conventions from email and usenet."

Word to Markdown Converter (online)

GitHub - lunet-io/markdig: A fast, powerful, CommonMark compliant, extensible Markdown processor for .NET

Ace - The High Performance Code Editor for the Web