Alex Mansfield

This plugin allows you to easily add code to your pages and posts without worrying about what the WordPress editor will do to it. You can embed HTML, video embed codes, Paypal forms, Google maps and more. Please note: this plugin will not allow you to run server side scripts such as PHP.
To use the plugin, all you have to do is create a custom field (you can name it anything), place your code in the new custom field, and place the following shortcode into your post where you want the code to go:
<pre><code>[easyembed field="name-of-custom-field"]
</code></pre>

Easy Embed

Allows any code to be easily embedded in posts and pages.

Felipe Lavín

This plugin adds an editor button that will let you use the great <a href="http://codemirror.net" rel="nofollow ugc">CodeMirror</a> editor to edit the HTML code of your posts.
This editor supports code coloring and automatic tag closing on a big editor that will let you write beautiful code on your WordPress posts.
Probably the best choice if you need to use complex HTML on your posts, or just enjoy writing in plain HTML.

CodeMirror for Post Editor

Write beautiful HTML code using the CodeMirror component for your WordPress posts.

Easy Embed vs CodeMirror for Post Editor