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

stanxp

This plugin will disable the visual editor for selected page(s)/post(s)/custom post types. The idea behind this came after i had to keep the html intact by the tinymce editor whenever i switched back to Visual tab in the editor.

Disable Visual Editor WYSIWYG

I've added this to my list of "Essential Plugins". Doesn't need to be by default, but it makes it wonderful for having multiple admins coordinating pages and making sure people know when they can/shouldn't rely on the WYSIWYG editor.

Reduces so much pain

This plugin will disable the visual editor for selected page/post..

MarkdownBar vs Disable Visual Editor WYSIWYG