I want to point out a few keyboard shortcuts in a place you might not expect to find them: the plain text editing side of one of my favorite writing tools, Byword.

XSLT stylesheet to transform XHTML into Markdown-formatted text

MarkdownPad is a full-featured Markdown editor for Windows.

Convert files from one markup format into another. Can read markdown and (subsets of) reStructuredText, textile, HTML, and LaTeX, and it can write plain text, markdown, reStructuredText, HTML, LaTeX, ConTeXt, PDF, RTF, DocBook XML, OpenDocument XML, ODT, GNU Texinfo, MediaWiki markup, textile, groff man pages, Emacs org-mode, EPUB ebooks, and S5 and Slidy HTML slide shows. PDF output (via LaTeX) is also supported with the included markdown2pdf wrapper script

Markdown implementation in javascript