MarkitUp (beta) is a wordpress plugin which allows you to use Markdown on your site without breaking existing content
MarkItUp (beta) allows for you to use Markdown on your WordPress site.
It does this on a per-author basis and per-post. This means that you shouldn't be accidently converting other people's posts/pages into Markdown by accident!
Currently this wears the beta label because it could have little bumps which aren't smoothed out just yet.