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.

Claudio Sanches

This plugin adds Github Gists in your posts via shortcode.
Examples
Add a full gist:
<pre><code>[gist id="3837669"]
</code></pre>
Add a specific file in a gist:
<pre><code>[gist id="3837669" file="index.php"]
</code></pre>
<h3>License</h3>
Gist GitHub Shortcode is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published 
by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.
Gist GitHub Shortcode is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of 
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
You should have received a copy of the GNU General Public License along with Gist GitHub Shortcode. If not, see <a href="http://www.gnu.org/licenses/" rel="nofollow ugc">http://www.gnu.org/licenses/</a>.

Gist GitHub Shortcode

Adds Github Gists in your posts via shortcode

GitHub-Flavored Markdown Comments vs Gist GitHub Shortcode