Mirte

If, for example, your clients want to be able to “turn off” their website for a while, so they can work on new 
content or features of their website, but you don’t want them rummaging through the WordPress Settings pages to do so, this plugin might be something for you.
This plugin creates a very simple little widget for your WP Dashboard, from which the website can turned to / out of maintenance mode.
Activating maintenance mode will display a maintenance mode page with explanatory text for your visitors. Logged in users will be able to view the website as they normally would.
The plugin will automatically create a <code>maintenance.php</code> file and add it to your <code>/wp-content/</code> directory. Customize this page to change the appearance and text of the maintenance mode page.
Known bugs 
* Right now, this plugin will not work on certain servers (like Antagonist.nl) because of a permissions issue.

Dashboard Maintenance Mode

Adds a nifty widget to your dashboard from which you can put your website in maintenance mode.

Noor Alam

<a href="https://wphowto.net/simple-maintenance-plugin-wordpress-595" rel="nofollow ugc">Simple Maintenance</a> plugin can be easily installed to show a maintenance mode page. It allows you to maintain your site on the back end by displaying a temporary maintenance page on the front end.
The plugin does not require any additional configuration or setup. All you need to do is activate it and maintenance mode will be automatically enabled. It is very lightweight yet powerful. It uses the HTML5 layout which makes the maintenance mode page viewable from a mobile device.
<h4>Features</h4>
<ul>
<li>Clean HTML and CSS layout</li>
<li>Responsive maintenance mode page</li>
<li>No customizations required</li>
<li>Easier to manage as the maintenance mode can be turned on by activating the plugin</li>
<li>Notify the search engine crawler that the site is down for a limited period of time</li>
</ul>
For more information please check the <a href="https://wphowto.net/simple-maintenance-plugin-wordpress-595" rel="nofollow ugc">WordPress Maintenance Mode Plugin</a> documentation page

Simple Maintenance

Really simple and to the point. Exactly what I was looking for. I just had to modify the message and the styles a bit.

Great plugin

Display a simple maintenance mode page while your site is undergoing scheduled maintenance

Dashboard Maintenance Mode vs Simple Maintenance