crimson090

Adds buttons to the Text (HTML) edit which generate Markdown syntax. These buttons serve as a great introduction to 
users who are new to Markdown, as well as shortcuts for more experienced users. The buttons include:
<ul>
<li>Bold</li>
<li>Italic</li>
<li>Blockquote</li>
<li>Link</li>
<li>Code</li>
<li>Table</li>
<li>Headers</li>
<li>Help (links to external Github Markdown guide)</li>
</ul>
This plugin does not modify the Visual editor, as that editor is not ideal for writing Markdown in. It’s recommended you disable 
the Visual editor entirely to enforce using Markdown.
Note that this plugin does not render Markdown. You should use a plugin (such as Jetpack Markdown) to render markdown in your theme.

MarkdownBar

Adds a toolbar of buttons to the Text (HTML) edit which generate Markdown syntax

Benjamin Chris

WP Editor.md is a beautiful and practical Markdown document editor.
Build support for the WordPress using Editor.md.
The plugin uses the Markdown module from WordPress <a href="http://jetpack.me" rel="nofollow ugc">Jetpack</a> for parsing and saving content.Thank for Jetpack modules.

MarkdownBar vs WP Editor.md – The Perfect WordPress Markdown Editor