ijmccallum

This theme demo / theme switcher plugin will add a bar to the top of your WordPress site listing every theme you have installed. It gives visitors the ability to switch between themes and try out their look on your live site.
After installation it requires no set up, it just works.
If you would like to see any extensions to this theme switcher please don’t hesitate to contact me from my website or post something in the support section. And, if you like it – please leave something nice in the review section! The more popular this becomes the more likely I’ll be to work on giving it lots more extra awesome.
<h4>Why another theme switcher?</h4>
I built this theme switcher primarily for myself. There are a number of other theme switchers out there but they all require visitors to click on a drop down menu then select a theme to demo or have some other complication and none of them had the clean, simple, minimal way of switching that I was hoping for in a theme switcher. So I built this theme switcher to give visitors an instant list of all the themes I hope to develop which they will be able to demo in a single click, also it doesn’t need any set up so it’s very easy!
So, I imagine most of the people interested in this theme switcher would be theme developers but if there are any people who are really indecisive about how they want their website to look, I guess this could provide them with an interesting solution to the website design problem. Let me know if you can think of any other uses for a theme switcher!

IJM Theme Switcher Bar

Add a theme switcher / theme demo bar to your site. Allows users to switch the theme they see on your site.

xhtmlweaver

In short, this plugin allows you to modify/switch the current theme with live site on the fly without messing up with your current visitors. 
This plugin is perfect for WordPress theme developers, freelancers as it allows you to edit the live theme without interfering the current visitors. 
It offers following features:
<ul>
<li>Ability to duplicate any current themes to the new theme for editing purpose.</li>
<li>Selectively enabling the theme switcher on the frontend by enabling extra settings in the backend. (By role or by IP Addresses)</li>
<li>Provides an Ajax theme switcher on the right top which selectively allows the visitor (by role or IP Addresses) to switch the theme.</li>
<li>A transparent banner on the top to display current theme.</li>
<li>Full Support is available at http://www.xhtmlweaver.com or wp-support@xhtmlweaver.com</li>
</ul>

Parallels Themes Switcher

This plugin allows you to modify/switch the current theme on the live site without interfering the current visitors.

IJM Theme Switcher Bar vs Parallels Themes Switcher