This is intended as a quick reference and showcase. For more complete info, see John Gruber's original spec and the Github-flavored Markdown info page.
This cheatsheet is specifically Markdown Here's version of Github-flavored Markdown. This differs slightly in styling and syntax from what Github uses, so what you see below might vary a little from what you get in a Markdown Here email, but it should be pretty close.
You can play around with Markdown on our live demo page.
(If you're not a Markdown Here user, check out the Markdown Cheatsheet that is not specific to MDH. But, really, you should also use Markdown Here, because it's awesome. http://markdown-here.com)