Interesting to see that 10 years after John Gruber came up with Markdown, there's only now any sort of standardization. Standard Markdown was just announced today. As someone who uses Markdown for just about everything (notes, presentations, blogs), I'm definitely glad to see this happening.
Edit: It looks like this was done without the blessing of John Gruber. Until things clear out, I'm removing any links to "Standard Markdown." I'm also fixing my earlier mistake of not linking to either John's site, Daring Fireball (which I read religiously), or his page dedicated to Markdown.