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.

dflydev

This project is available for forking on GitHub:
<ul>
<li>https://github.com/dflydev/embed-github-gist</li>
</ul>
Embed <a href="http://github.com/" rel="nofollow ugc">GitHub</a> <a href="http://gist.github.com" rel="nofollow ugc">Gists</a> into 
WordPress. Provides a shortcode for posts and pages but also has the ability 
to embed by hand in the event that a Gist needs to be embedded somewhere in 
the page that does not pass through the shortcode filters.
Examples:
<pre><code>[gist id=546764]
[gist id=546764 file=file.txt]
[gist id=546764 file=file.txt bump=1]
[gist]http://gist.github.com/546764[/gist]
</code></pre>
Cache is implemented with the Transients API to minimize delay on loading 
content. Default TTL (time to live) is 86400 seconds or one day.
<h4>Upcoming features:</h4>
<ul>
<li>Option for setting default TTL</li>
<li>Option to bypass cache entirely</li>
<li>Implement admin interface to control options</li>
</ul>

GitHub-Flavored Markdown Comments vs Embed GitHub Gist