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.

k0tik

The usual Text widget allows you to insert arbitrary Text and/or HTML code. This allows that too, but also parses any PHP or JavaScript code in the text widget and executes it.
This plugin is based on “PHP Code Widget”, but now available for use by site administrators ONLY, which makes it more secure.
All PHP code must be enclosed in the standard php opening and closing tags ( <code><?php</code> and <code>?></code> ) for it to be recognized and executed. Also JavaScript code must be enclosed in the <code><script></code> and <code></script></code> tags, as usual.

Safe PHP Code Widget

Adds a secure and simple widget in which you can use PHP and JavaScript code. Also you can use unfiltered HTML or just Text. Admin Use Only.

Easy Embed vs Safe PHP Code Widget