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.

upwerd

Remove or replace the footer credit of any WordPress theme with the click of a mouse.

Visual Footer Credit Remover

Remove or replace footer credit of any WordPress theme with the click of a mouse.

nuAdmin Footer vs Visual Footer Credit Remover