How to set up a demo?

Hey all, well I own a blog lets just say it's x.com I have been to many sites where the blog is installed on x.com and they have demos of other themes they have made such as "x.com/theme=xxx" I did a bit of a google search but could not find anything, does anyone know of a plugin or a way to do this?

 

This is what you are looking for http://dev.wp-plugins.org/wiki/ThemeSwitcher...

 

thanks i'll check it out.