Ability to enter maintenance lockdown mode: force-logout all non-administrators, prevent logins from non-administrators, and disable commenting.
Sometimes you don't need (or want) to shut down the entire frontend of the site during maintenance. Unlike the default WordPress maintenance mode, this plugin allows your site to stay running.
This plugin does not interact with, prevent, or override WordPress's maintenance mode. It's complementary.
WordPress.org plugin repository: http://wordpress.org/extend/plugins/pmc-lockdown/
Image by Chris Randall; some rights reserved. See link for details.