Add a maintenance page to your blog that lets visitors know your blog is down for maintenance, add a coming soon page for a new website or create a landing page for an existing site. User with admin rights gets full access to the blog including the front end.
Activate the plugin and your blog is in maintenance-mode, works and only registered users with enough rights can see the front end. You can use a date with a countdown timer for visitor information or set a value and unit for information.
Also works with WordPress Multisite installs (each blog from the network has its own maintenance settings).