Markdown by DaringFireball - Text-to-HTML conversion tool/syntax for web writers, by John Gruber
CKEditor - Real-time collaborative future-ready rich text editor
GitHub Flavored Markdown - GitHub uses "GitHub Flavored Markdown," or GFM, across the site--in issues, comments, and...
TinyMCE - TinyMCE is a content editor that functions as a plug-in for Wordpress websites.
StackEdit - Full-featured, open-source Markdown editor based on PageDown, the Markdown library used by Stack Overflow and the other Stack Exchange sites.
Trumbowyg - Feature-packed WYSIWYG editor in only 15kB of JavaScript