dubhunter

WordPress’s themes are absolutly amazing, except you are not able to theme the login pages very well. 
DBD Login Style is a light weight plugin that will allow you full theme control over the login pages. 
It also offers additional DIV tag creation for custom messages.
Some Features:
<ul>
<li>Specify CSS style sheet for login pages.</li>
<li>Change the URL and TITLE of the header link.</li>
<li>Add specific messages to the different login pages (Login, Registration, Forgot Password)</li>
<li>Change various error messages on login and registration pages.</li>
</ul>

DBD Login Style

Add style to your login page!! This plugin enables you to specify a style sheet to be used on the login page.

Travis Smith

WordPress’s themes are absolutly amazing, except you are not able to theme the login pages very well. Smithers Login is a light weight plugin that will allow you full theme control over the WordPress Multisite login pages. This plugin allows for different custom login screens on a single WordPress 3.0 Multisite with Domain Mapping. This Network Activated plugin enables you to specify a stylesheet to be used with every multisite login page allowing for custom login images and messages.
Some Features:
<ul>
<li>Specify CSS style sheet for login pages.</li>
<li>Change the URL and TITLE of the header link.</li>
<li>Add a custom image to the login page with PSD file.</li>
<li>Add specific messages to the different login pages (Login, Registration, Forgot Password)</li>
<li>Change various error messages on login and registration pages.</li>
<li>Enhances and extends DBD Login Style.</li>
</ul>

Smithers Login

Add style to your WordPress multisite login pages! This plugin enables you to specify a CSS to be used on the login page with a custom image/messages.

DBD Login Style vs Smithers Login