AJ Troxell

With the Custom Login Page, you can change the background image of your login page, the background image/colo of your login form div, the main logo image of your login page, and add custom CSS. 
All through a simple interface. Remember to view the Help tab at the top of the settings page for the plugin.

Custom Login Page

Wordpress Custom Login Page allows you to change the background image, logo, form div attributes, and custom css of you Wordpress login page.

Vast HTML

This is a full featured plugin to help you edit your login just the way you want it. Lets you use or block the original wordpress 
css (the first plugin to do this). Comes with one pre made style you can use or deactivate. 
Very easy to create your own style using the style tags givin in the plugin. 
Please visit http://vasthtml.com/js/login-xchange-plugin/ for usage instructions.

Login XChange

This plugin will let you change the css of your login and register pages.

Custom Login Page vs Login XChange