Gruber's original presentation of Markdown. It includes Dingus a tool for seeing Markown in action. Dingus has a hidden feature I love: It will intelligently remove carraige returns from documents that have them at the end of very line, while preserving existing paragraph breaks.
Web content creation tools
From Wikipedia: The Markdown language was created in 2004 by John Gruber with substantial contributions from Aaron Swartz,with the goal of allowing people “to write using an easy-to-read, easy-to-write plain text format, and optionally convert it to structurally valid XHTML (or HTML)”
This is an extremely handy tool for writing web content – created by famed Mac blogger John Gruber (Daring Fireball). And when it's used with some of the other tools that have been created to work with it, it's both a powerful and easy way of preparing web content.
MOU is a Markdown editor that shows you the markdown text in one panel and a preview of the formatted text in another – quite useful for checking your work as you go.
Marked is a preview tool with the abilty to export in HTML, PDF, Paginated PDF, RTFD, RTF, DOC, DOCX, and ODT.