kennydude

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.

MarkItUp!

MarkitUp (beta) is a wordpress plugin which allows you to use Markdown on your site without breaking existing content

Dev7studios

Typewriter completely removes the “Visual Editor” feature in WordPress and replaces it 
with a simple Markdown editor so that you can join sites like StackOverflow and Github in writing in the 
simple and growingly popular <a href="http://daringfireball.net/projects/markdown" rel="nofollow ugc">Markdown format</a>.
For the more technical among you, Typewriter removes the TinyMCE editor and defaults to a Markdown specific “Text” editor 
and parses the output using MarkdownExtra.

Typewriter

Typewriter replaces the Visual Editor with a simple Markdown editor for your posts and pages.

MarkItUp! vs Typewriter