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.

kartikaykanojia

A code syntax highlighter an easy to use source code embedding in wordpress with different(55) themes with 172 languages. 
It uses dynamic script loading to load only essential javascript to render code syntax highliting. 
It uses most powerfull open source library CodeMirror as engine.
Features
<ul>
<li>Dynamic script loading to minimize load time and http call</li>
<li>172 language support</li>
<li>172 shortcodes</li>
<li>Shortcode picker in editor</li>
<li>Lightweight language selector</li>
<li>Multiple Theme support</li>
<li>55 Themes</li>
<li>Theme manager</li>
<li>Theme uploader (Upload your own theme)</li>
<li>Compatible with all WordPress themes</li>
<li>Fast and secure</li>
<li>Cross browser compatible</li>
<li>Line numbering</li>
<li>A powerful, composable language system</li>
<li>Bracket and tag matching</li>
<li>Line selector</li>
<li>Able to resize to fit content</li>
</ul>
Minimum system requirements:
<ul>
<li><a href="https://php.net" rel="nofollow ugc">PHP</a> version 5.2 or higher.</li>
<li><a href="https://www.mysql.com" rel="nofollow ugc">MySQL</a> version 5.0 or higher.</li>
</ul>
Recommended system requirements:
<ul>
<li><a href="https://php.net" rel="nofollow ugc">PHP</a> version 7.0 or higher.</li>
<li><a href="https://www.mysql.com" rel="nofollow ugc">MySQL</a> version 5.6 or higher.</li>
</ul>
Credits
<ul>
<li>The banner of plugin is a copyrighted image created by the <a href="http://www.briskjab.com" rel="nofollow ugc">Briskjab</a>. (C) All rights reserved.</li>
<li><a href="https://codemirror.net/" rel="nofollow ugc">CodeMirror</a> is an open-source project shared under the <a href="https://codemirror.net/LICENSE" rel="nofollow ugc">MIT license</a>.</li>
</ul>

Code Syntax Highlighter

A simple dynamic loading syntax highlighter.Supports 172 languages, 172 shortcodes and 55 themes.

WordPress Code Snippet vs Code Syntax Highlighter