WhizKins

WPSimpleInsert is a plugin to insert custom code in your blog posts. It includes options to either add the code above 
or below post content.
Features:
<ul>
<li>Add custom HTML/PHP/JavaScript to your posts.</li>
<li>Ability to add custom code above or below post content.</li>
<li>Options to hide the code from certain pages like the home page and category and archives pages.</li>
</ul>

WP Simple Insert

A plugin to insert custom HTML or PHP code in your posts. Includes options to insert before or after post content.

John Godley

Sniplets is a generic text insertion plugin with support for an extensible processing framework. At it’s simplest this means you can dynamically replace text in your posts with text that may be defined elsewhere, or created by some other module. For example, you can use Sniplets to perform syntax highlighting of files, execute custom PHP code, insert data from a database, and perform all manner of other useful tasks within a standard interface.
<ul>
<li>Replace tags in a post with pre-defined text</li>
<li>Apply processing functions to the pre-defined text to perform additional functionality</li>
<li>Automatic insertion – data can be made to automatically appear in various places (header, footer, before post, after post, more tag, initialization)</li>
<li>Custom processors can be written by the user</li>
<li>Fully localized</li>
</ul>
Sniplets is available in:
<ul>
<li>English</li>
<li>Belorussian thanks to Marcis Gasuns</li>
</ul>
<h3>Documentation</h3>
Full documentation can be found on the <a href="http://urbangiraffe.com/plugins/sniplets/" rel="nofollow ugc">Sniplets</a> page.

Sniplets

Sniplets is a generic text insertion plugin.  Use it to manually or automatically insert data, PHP, syntax highlight, and almost anything else!

WP Simple Insert vs Sniplets