I like to Hotedote

GitHub supports many extras in Markdown that help you reference and link to people. If you ever want to direct a comment at someone, you can prefix their name with an @ symbol: Hey @kneath — love your sweater!

But I have to admit, tasks lists are my favorite:

• This is a complete item
• This is an incomplete item

When you include a task list in the first comment of an Issue, you will see a helpful progress bar in your list of issues. It works in Pull Requests, too!

And, of course emoji!

function fancyAlert(arg) {
  if(arg) {
    $.facebox({div:'#foo'})
  }
}