This bookmarklet gives you a markdown editor/previewer in your browser in a single click.

Markdown Preview Bookmarklet

data:text/html, <script type="text/javascript" src="https://raw.github.com/MrMitch/downatello/develop/downatello.js"></script><style type="text/css">#md,#html{position:absolute;top:0;bottom:0;padding:5px;border:none;outline:none;font-family:monaco,consolas,sans-serif;font-size:1em}#md{resize:none;left:0;right:50%;border-right:1px solid black}#html{left:50%;right:0;font-family:sans-serif}</style><textarea id="md" oninput="document.getElementById('html').innerHTML=downatello.toHtml(document.getElementById('md').value);"></textarea><div id="html"></div>

Thanks for https://coderwall.com/p/lhsrcq and https://gist.github.com/4666256 for the idea.
The bookmarklet is using MrMitch/downatello javascript markdown parser.

Doesn't work with Chrome. Nor does https://otaqui.com/blog/1771/note-markdown-live-preview-bookmarklet/index.html