As you may know, you can set your Joomla site "offline" by setting "Site Offline" to "Yes" in the Global Configuration. This is a great feature that enables you to build or troubleshoot your site in offline mode, restricting access only to backend users who login through the login form displayed on the offline page.
The only limitation to this feature is that you have to be logged in to see the frontend of the site, so you cannot test the site from the perspective of a user who is not logged in. For example, if you want to test registration, AEC or Ambra Subscriptions integration, or any number of other things that might require you to see the site from the perspective of a non-logged-in user, you cannot do that because offline mode requires you to log in to see the front end of the site.
I have developed a Joomla plugin called Offline Override that enables you to put the site in offline mode, and enter a keyword to override offline mode without requiring a login. You simply install and enable the Offline Override plugin, create your secret keyword (no spaces) and enter it in the plugin's parameters. Then, when your site is in offline mode, you can override the offline mode by adding "?keyword=VALUE" in the address bar for the frontend of your site. For example, if your keyword is "joomlarocks", you would go to http://yoursite.com/?keyword=joomlarocks. The override only lasts the length of your session and you must re-enter the keyword in the address bar (http://yoursite.com/?keyword=joomlarocks) the next time you open your browser.