by Fugu Design on WordPress.org
Customize easily and switch in one-click to (native) maintenance mode from your backend or frontend.

A button is added to the admin bar for simple toggling in maintenance mode.
One-click maintenance mode
This plugin adds a button to the admin bar for toggling the builtin maintenance mode.
Core maintenance mode
A .maintenance file is generated and copied to the WordPress installation folder when turning on the maintenance mode.
A maintenance.php file is generated and added to the wp-content folder for custom HTML code.
Your custom maintenance page will also be active during the core, plugins and themes updates.
set which roles can switch the maintenance mode
set which roles can bypass the maintenance mode on the frontend
set the entire HTML code used for the maintenance page
preview the maintenance page before saving
enable the theme file support, so you can create in each your themes a maintenance.php file to customize the maintenance page
restore all default settings
restore default HTML code
create/delete the maintenance file in the active WP theme