by jamiewilson on WordPress.org
Gatekeeper allows administrators to take a WordPress site offline while leaving it accessible to authorized users for maintenance, testing, etc.

The Gatekeeper administration page
Gatekeeper allows administrators to take a WordPress site offline quickly and easily while leaving it fully accessible to administrators and other authorized users. Site visitors will be shown or redirected to a specified offline page. An optional blacklist can be used for permanent bans.
** Features **