Evan Solomon

Comment forms allow some HTML, this plugin extends that to let users type in <a href="http://daringfireball.net/projects/markdown/" rel="nofollow ugc">Markdown</a> and have it automatically converted to HTML. It also extends Markdown with a couple features made popular <a href="https://github.com/github/github-flavored-markdown" rel="nofollow ugc">by GitHub</a>. Conversion from Markdown to HTML is completely automated and doesn’t require anything on the commenter’s part.
The feature additions from GitHub are:
<ul>
<li>Single linebreaks are treated as new paragraphs</li>
<li>Code “fencing” with three backticks (“`)</li>
</ul>
Two notable features from GitHub’s Markdown implementation are missing:
<ul>
<li>To do lists: Doing these well would require some front end interaction, which I decided is outside the scope of this plugin at least for now</li>
<li>Repository autolinking (e.g. comments, issues, etc): These seemed a little too GitHub-specific.</li>
</ul>
GitHub-Flavored Markdown Comments is developed, unsurprisingly, <a href="https://github.com/evansolomon/wp-github-flavored-markdown-comments" rel="nofollow ugc">on GitHub</a>.

GitHub-Flavored Markdown Comments

WordPress plugin to let commenters use (GitHub-flavored) Markdown, and turn it into HTML.

Stephen Harris

This plugin allows you to write posts (of any post type) using the Markdown syntax. The plugin converts the Markdown into HTML prior to saving the post. When editing a post, the plugin converts it back into Markdown syntax.
The plugin also allows you to enable Markdown in comments and bbPress forums. In these instances the plugin adds a toolbar, and preview of the processed Markdown with <a href="http://code.google.com/p/google-code-prettify/" rel="nofollow ugc">Prettify</a> syntax highlighter applied (similiar to that used in the Stack Exchange websites such as <a href="http://wordpress.stackexchange.com/" rel="nofollow ugc">WordPress Stack Exchange</a>).
WP-Markdown stores the processed HTML, so deactivating the plugin will not affect your posts, comments or bbPress forums.

WP-Markdown

Version 1.5.1 of the plugin works almost perfectly.
I'm just waiting for the upcoming 1.6 update (which is in alpha on GitHub) to be pushed, so I can use GitHub Flavor-style codeblocks specifying the exact language.



Waiting for 1.6

Allows Markdown to be enabled in posts, comments and bbPress forums.

GitHub-Flavored Markdown Comments vs WP-Markdown