Offline Override 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.