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.

Jan Štětina

This plugin allows to edit and store multiple PHP scripts within WordPress administration (Tools -> Admin PHP Eval) and execute them repeatedly with <code>eval()</code>. After evaluation the return value and echoed data is displayed.

Admin PHP Eval

Storing and evaluating PHP scripts within WordPress administration.

PHP-Widgetify vs Admin PHP Eval