ibnuyahya

This plugin allow you to display related post in the sidebar. It’s only appear when you open a single post. You can choose whether want to list all post or limit by number of posts.
<ul>
<li>Author: ibnuyahya</li>
<li>Author URI: <a href="http://www.ibnuyahya.com" title="developers note" rel="nofollow ugc">http://www.ibnuyahya.com</a></li>
<li>Copyright: Released under GNU GENERAL PUBLIC LICENSE</li>
</ul>

SKSDEV

Allow PHP Execute Run PHP code inserted into WordPress posts and pages and Text Widget 
also you can add php code in your post or page title section.
The PHP code runs as the page is sent to the browser. Output of the PHP code is published 
 directly onto the post , page or widget section where the PHP code is located. 
you can be activated without any fear.
Examples of use:
<ul>
<li>Use the php code on your page, post ,default text widget ,page title or post title
</li>
</ul>

Allow PHP Execute

Executes PHP code on WordPress post ,page and on default Text Widget

Related post widget vs Allow PHP Execute