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.

António Campos

With this plugin you can easily choose one image from your gallery and use it as logo in your wp-login.php

AC Change Login Image

This plugin changes the image in the login page (wp-login.php), you can choose any image from your gallery and use it as login logo.

Login XChange vs AC Change Login Image