Brad Williams

This plugin will display all themes located in wp-content/themes on a page or post in a showcase gallery with theme screenshots and preview links. 
This is a great plugin for designers looking to showcase their work or anyone that wants an easy way to display and preview multiple themes.
New settings page allows you to choose which themes are displayed
For support visit our Theme Showcase Support Forum: 
http://webdevstudios.com/support/forum/wordpress-theme-showcase-plugin/
Features:
<pre><code>* Display a showcase of your themes
* Theme screenshots are displayed with titles and descriptions
* Easily preview all themes loaded in WordPress
* Allow anyone to preview themes, not just administrators
* Works with both WordPress 2.x and WordPress MU (2.7 approved!)
* New settings page lets you pick which themes are displayed
</code></pre>
<h3>Plugin Support</h3>
<a href="http://webdevstudios.com/support/forum/wordpress-theme-showcase-plugin/" title="WordPress Plugins" rel="nofollow ugc">WordPress Theme Showcase Support</a>

WordPress Theme Showcase Plugin

Display themes located in wp-content/themes on a page or post in a showcase gallery with theme screenshots and preview links.

rabby

Let the wpmu users select a theme when registering. This plugin should also work with buddypress extended WPMUs. By default the number of themes (screenshots) per line and the number of rows is set to 3. You can simply change it in the .php where it can be found: $n_per_line=3; $n_rows=3; 
Note: This is open source with free incommercial usage. For commercial projects, please contact ahammerl[at]googlemail/dot/com. Steeling open source, is illegal Software Theft. 
Feb 2010: We worry about homehost.com (powered by pointaxis.com) which is using the script without permission and violating the terms.

WPMU Theme Select

Let the wpmu users select a theme when registering

WordPress Theme Showcase Plugin vs WPMU Theme Select