Allan Collins

This plugin will allow you to add code snippets to your pages/posts. This is great for code tutorial sites.
<h3>Upgrading</h3>
If you are upgrading from version 1.0, you will see an option on the Code Library page that will allow you to upgrade. As a precautionary measure, be sure to back up your database before doing so.

WordPress Code Snippet

This plugin will allow you to add code snippets to your pages/posts.  This is great for code tutorial sites.

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

WordPress Code Snippet vs Gist GitHub Shortcode