Visit the website.
Get it for Chrome.
Get it for Firefox.
Get it for Safari.
Get it for Thunderbird and Postbox.
Get it for Opera.
Discuss it and ask questions in the Google Group.
Markdown Here is a Google Chrome, Firefox, Safari, Opera, and Thunderbird extension that lets you write email† in Markdown‡ and render them before sending. It also supports syntax highlighting (just specify the language in a fenced code block).
Writing email with code in it is pretty tedious. Writing Markdown with code in it is easy. I found myself writing email in Markdown in the Github in-browser editor, then copying the preview into email. This is a pretty absurd workflow, so I decided create a tool to write and render Markdown right in the email.
To discover what can be done with Markdown in Markdown Here, check out the Markdown Here Cheatsheet and the other wiki pages.
†: And Google Groups posts, and Blogger posts, and Evernote notes, and Wordpress posts! See more.
‡: And TeX mathematical formulae!