TextDown is a simple but well designed and customizable text editor for Google Chrome that supports the Markdown format including Github flavored markdown.
You can either use the Markdown syntax to format your text or use one of the many keyboard shortcuts (like Ctrl+B for bold). It also supports abbreviations for quickly writing commonly used words and phrases (like your email address or phone number).
While editing inside Textdown, you can press Ctrl +P to preview your text in rich HTML. Textdown is open source and its design was inspired by the popular Byword editor for Mac.