Josh Pollock

Use this plugin to create a demo site for your themes, or use it to allow users to customize the presentation of your site.
The JP Theme Bar Plugin adds a theme switching bar to the bottom of your site, perfect for theme preview sites. The settings page for the plugin lets the end user choose which themes to add, as well as set the colors for the theme bar. You can see it in action on this site.
New in version 0.0.3 -> Themes will update theme mods when changing themes allowing for individual theme settings to be previewed properly.
This plugin is based on the <a href="http://iainjmccallum.com/wordpress/live-demo-theme-bar/" rel="nofollow ugc">IJM Theme Switcher Bar v2.0</a> by <a href="http://www.iainjmccallum.com/" rel="nofollow ugc">Iain J McCallum</a>
IMPORTANT- The plugin <a href="https://wordpress.org/plugins/theme-test-drive/" rel="ugc">Theme Test Drive</a> by <a href="http://www.prelovac.com/vladimir/" rel="nofollow ugc">Vladimir Prelovac</a> must be installed and activated in order for the theme switching to work.

JP Theme Switcher Bar

Adds a theme switcher/ theme demo bar to the bottom of your site to allow users to switch the theme they see on your site.

webbu

You can install Webbu Mobile Framework Theme Switcher (WMF Theme Switcher) plugin into your desktop website and choose another theme for mobile devices without affecting your desktop theme.
Detects all the Mobile Browsers and loads the mobile theme per your settings done in the admin.
<ul>
<li>iPhone</li>
<li>iPad</li>
<li>Android</li>
<li>Windows Mobile</li>
<li>Blackberry</li>
<li>Other Tablet Browsers</li>
<li>Other Mobile Browsers</li>
</ul>
You can select a different mobile theme for each mobile browser. You can select different mobile themes for each of the options above.
Webbu Mobile Framework Theme Switcher using <a href="http://mobiledetect.net/" title="mobile detect" rel="nofollow ugc">Mobile Detect</a> library to detect devices.

WMF Theme Switcher

This plugin switch themes per mobile and tablet device. Useful for the switch to a mobile theme.

JP Theme Switcher Bar vs WMF Theme Switcher