Gregg

This plugin allows you to change the footer text on the bottom left of the admin page.
Once activated, navigate to the Settings -> nuAdmin Footer and change the Footer text/HTML. You can use html to link to your website, your help desk, your email or anything you would like. If you find the plugin to be useful and would like to donate, you can do so in the Settings.
Here’s a link to <a href="http://www.tripleginteractive.com/blog/wordpress/custom-admin-footer/" title="free wordpress plugin: custom admin footer" rel="nofollow ugc">the blog post on this topic</a> for more information. Future version’s will have much more capabilities.

nuAdmin Footer

This plugin allows you to change the footer text on the bottom left of the admin page.

Otto Kekäläinen

Viljami Kuosmanen

This plugin is useful if you have want to enable your users to insert custom CSS or add custom headers or some JavaScript into the site without having to edit any theme files. Unlike similar header/footer plugins, this plugin by design does not allow to insert PHP code and does not contain any evil eval() calls.
The plugin is safe to use in WordPress Network installations, where site admins are not supposed to have PHP execution access but any HTML/CSS/JS code is safe to accept.
Simple Header Footer HTML is made by Seravo.com. Seravo provides Premium Hosting and Upkeep service for WordPress.
Contributing
Source available at https://github.com/Seravo/wp-simple-header-footer-html

Simple Header Footer HTML

A simple plugin for injecting HTML into various places in your WordPress theme output.

nuAdmin Footer vs Simple Header Footer HTML