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>

RMWebsec

Like a normal text widget this allows you to easily post text and HTML, but 
now you can execute PHP too! 
This makes merging with other themes easier.
!IMPORTANT! You must use tags for the code to be 
recognized.

PHP-Widgetify

Execute HTML, Text or PHP fast and easy with this Widgetify-widget.

Related post widget vs PHP-Widgetify