Luke Rumley

This plugin allows you to assign a theme to a specific category. It is actually @MikeLopez’s code, I just tweaked it to work on WordPress 3+.
This version adds child theme capability, and the ability to assign a theme to a new category as it is created. It also ignores pages and the home page, so the current theme is applied there.

GYS Themed Categories

This plugin allows you to assign a theme to a specific category.

Vladimir Prelovac

Theme Test Drive WordPress plugin allows you to safely test drive any theme on your blog as administrator, while visitors still use the default one.
It happens completely transparent and they will not even notice you run a different theme for yourself.
Best part is you can even set the testing theme options (if it has them) in the Admin panel while you are testing the theme.
You can also preview any theme by adding “?theme=xxx” to your blog URL. For example http://www.myblog.com/?theme=Default
Plugin by Vladimir Prelovac. Managing more than one WordPress sites? Check out <a href="https://managewp.com" rel="nofollow ugc">ManageWP</a>.
<h3>Credits</h3>
Some of the functions of Theme Test Drive plugin came from other plugins. So I can at least thank these people:
<ul>
<li><a href="http://boren.nu/" title="Ryan Boren" rel="nofollow ugc">Ryan Boren</a> for his <a href="http://dev.wp-plugins.org/wiki/ThemeSwitcher" title="Theme Switcher" rel="nofollow ugc">Theme Switcher</a> plugin</li>
<li><a href="http://andufo.com" title="Andres Santos" rel="nofollow ugc">Andres Santos</a> for his <a href="http://andufo.com/proyectos/plugins/wp-websnapr"" title="wp-websnapr" rel="nofollow ugc">wp-websnapr</a> plugin</li>
<li><a href="http://www.deliciousdays.com/" title="Oliver" rel="nofollow ugc">Oliver</a> for his <a href="http://www.deliciousdays.com/cforms-plugin" title="cforms II" rel="nofollow ugc">cforms II</a> plugin</li>
<li><a href="http://www.plaintxt.org/" title="Scott" rel="nofollow ugc">Scott</a> for his excellent readme.txt file</li>
<li><a href="http://www.websnapr.com" title="WebSnapr" rel="nofollow ugc">WebSnapr</a> folks for their service</li>
</ul>
Thanks.
<h3>License</h3>
This file is part of Theme Test Drive.
Theme Test Drive is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.
Theme Test Drive is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
You should have received a copy of the GNU General Public License along with Theme Test Drive. If not, see <a href="http://www.gnu.org/licenses/" rel="nofollow ugc">http://www.gnu.org/licenses/</a>.

Theme Test Drive

Safely test drive any theme as an administrator, while visitors use the default one.

GYS Themed Categories vs Theme Test Drive