Samuel Mello

This plugin was designed to create virtual micro-sites that employ different themes while using the same content from one source. It uses WordPress’s rewrite rules to decide which theme to display to the end user based off of the url they visit. This will allow you to access the same website in a virtual sub-directory with a completely different theme displayed.
REQUIRES PRETTY LINKS TO BE TURNED ON

Cameleon

Virtual Microsite Creator & Multi-Theme Enabler for WordPress

Masahiro NAKASHIMA

The theme can be changed and displayed to only logged in users.
The visitors of the website see your old theme while you are testing the new one.
Only the administrator of the WordPress site sees the new theme that can now be tested safely.
You can also use this plugin instead of “Theme Test Drive” plugin.

WP Theme Test

I have xdebug installed. There are two big Notices.

Threw two big PHP notices

The theme can be changed and displayed to only logged in users.

Cameleon vs WP Theme Test