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.

karevn

Themed login makes it possible to change the look of default WordPress login system using 
templates located at your theme folder, not just CSS-level customizations:
<ul>
<li>Login – login.php</li>
<li>Lost password form – retrievepassword.php</li>
<li>Registration – register.php</li>
<li>Registration confirmation – registered.php</li>
</ul>
If your theme does not contain one of the files described above – that is not a big deal, 
WordPress’s defaults will be used. See usage examples in “examples” plugin directory.
<h4>Plugin’s Official Site</h4>
<a href="http://karevn.com" rel="nofollow ugc">karevn.com</a>

Themed login

Makes WordPress login, registration and password retrieval pages totally themeable using WP theme mechanism.

Login XChange vs Themed login