Vitalii Kiiko

(Eng) Adds a “Maintenance Mode” page to your site (sending a “503 Service Unavailable” status). Includes MailChimp mail subscriptions.
(Ukr) Плагін додає сторінку-заглушку типу “На реконструкції” (надсилається статус “503 Сервіс недоступний”). Включає форму підписки на ел. пошту з допомогою MailChimp.
<a href="http://demo.uni-theme.net/" rel="nofollow ugc">Live demo!</a>
Features:
<ul>
<li>Used custom fonts with cyrillyc characters</li>
<li>Social service integration</li>
<li>MailChimp e-mail subscription form</li>
<li>2 themes available</li>
<li>Custom logo</li>
</ul>

Uni-theme Maintenance Mode

Fugu Design

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.
<h4>Special Features</h4>
<ul>
<li>
set which roles can switch the maintenance mode
</li>
<li>
set which roles can bypass the maintenance mode on the frontend
</li>
<li>
set the entire HTML code used for the maintenance page
</li>
<li>
preview the maintenance page before saving
</li>
<li>
enable the theme file support, so you can create in each your themes a maintenance.php file to customize the maintenance page
</li>
<li>
restore all default settings
</li>
<li>
restore default HTML code
</li>
<li>
create/delete the maintenance file in the active WP theme
</li>
</ul>
<h3>Translations</h3>
<ul>
<li>English – default, always included</li>
<li>French</li>
</ul>

Maintenance Switch

Customize easily and switch in one-click to (native) maintenance mode from your backend or frontend.

Uni-theme Maintenance Mode vs Maintenance Switch