How to Create a WordPress Theme Viewer
For WordPress veteran this might not be new, which is, let visitor preview multiple themes in one WordPress installation. It puzzled me quite a while before I figured out how to do it. The very reason for the need of this feature is that I want to allow regular users to view different themes on a WordPress powered website. I wasn't able to until I ran into this page. Yea right! A theme switcher. I installed this plugin and tested. It provides the exact function that I need. Fantastic.
Now here comes the interesting stuff. The official documentation documented the plugin is as follow, which outputs a list of links that can switch to a specific theme:
<li>Themes:
<?php wp_theme_switcher(); ?>;
</li>
If you view HTML source of those links, you'll find something similar to this:
index.php?wptheme=THEME_NAME
This is very useful because now I can build a WordPress theme viewer by this plugin alone. Let's say you have a number of WordPress themes installed and you want to show a demonstration of each to your visitors with a text link Test Run. You can easily accomplish this by appending the name of the theme to the wptheme parameter:
<a href="/?wptheme=THEME_NAME">Test Run</a> or <a href="/index.php?wptheme=THEME_NAME">Test Run</a>
The question now is how to find out the name of the theme. Easy enough, just log into Admin area and go to Presentation tab, where you can see a detail list of themes including their names. Even easier, under each theme folder, you can find a file called style.css, in which you can find theme name of the theme there.
You've just created a WordPress theme viewer.

(As for the question asked by the spam commenters, it's a legitimate one and the answer is to install Wordpress on your computer itself.)
Preview:
http://www.elegantthemes.com/preview/eVid/
Download:
http://www.sendspace.com/file/6uv0n2
Sample: http://wordpressthemesbase.com/
What plugin are they using?