The Demo Site plugin displays a simple notice to visitors in either Joomla! frontend or Joomla! Administrator - something very simple, that is actually very helpful if you're putting up a lot of different demo-websites.
When you put a demo-site live, you need to consider various things. Ofcourse the security aspect is vital: You need to make sure users can only access those parts of Joomla! that are vital, protecting other parts. One high risk is that you leave open the Joomla! Media Manager - a hacker could easily misuse this by uploading various scripts.
This plugin - as of yet - only offers a simple message that is displayed on the top of the screen. However, our goal is to add more security countermeasures as well to this plugin.