Penflip - Typewrite is a real-time collaborative writing environment with Markdown support, versioning and Dropbox syncing. Alternatives to Penflip for Web, Windows, Mac, Linux, iPhone and more.
Doxygen - Generate documentation from source code
Cleartext - A text editor that allows only the 1,000 most common words
DocFX - A documentation generation tool for API reference and Markdown files!
Quill - Powerful, API-driven rich text editor
Natural Docs - Natural Docs is an open-source documentation generator for multiple programming languages.