the best markdown editor BY FAR is IPython Notebook. What other markdown engine will render math (mathjax) and do syntax highlighting for you. I ended up grabbing all their js for a markdown-based blog platform I built myself

another neat markdown hack is to get the quicklook markdown renderer (if you're on os x) and then use copy in the github markdown CSS. you can see immediately what you're README.md files are gonna look like right away in finder