Chrome/Firefox/Safari/Thunderbird/Postbox: Markdown is a great way to instantly render text as HTML without knowing a lick of coding. If you want to use that in an email client like Gmail or Postbox, Markdown Here adds the functionality easily.
With Markdown Here installed, you simply write out a block of text in Markdown, select it, and then click the Markdown Here button. The extension then converts that text into HTML instantly. It works in most email clients, as well as other web editing interfaces including Evernote, Wordpress and many more. If you're a fan of Markdown, Markdown Here is a really handy extension to keep around.