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.

WPExperts.io

Maintenance mode or a coming soon page with progress bar and responsive layout. Simply enable the maintenance mode from the plugin settings and in few seconds you can put your site in maintenance mode.
With a progress bar and countdown timer visitors will be informed about the time left till launch and you can collect their emails using built-in MailChimp integration.
Features
<ul>
<li>Simple & easy to use interface – perfect for non tech users</li>
<li>Create beautiful maintenance mode or coming soon page for your WP site</li>
<li>Change heading & sub-heading</li>
<li>Set custom title for maintenance mode or coming coon page</li>
<li>SEO Optimized</li>
<li>Enable / Disable maintenance or coming soon mode</li>
<li>Progress Bar</li>
<li>Countdown Timer</li>
<li>Visual editor for changing content</li>
<li>Social Icons</li>
<li>Upload your own logo</li>
<li>Change background color and various text colors</li>
<li>Choose from hundreds of fonts</li>
<li>Set SEO Title of maintenance mode page</li>
<li>Set Favicon</li>
</ul>

Rocket Maintenance Mode & Coming Soon Page

Easy to use and almost fully customizable



Does what it says - creates a maintenance page, but everything is pretty much locked down unless you upgrade to the premium version, which the plugin authors annoyingly let you know everywhere. Makes you think that they're just trying to make a buck. Can't even change the font color.
There are better plugins that do exactly this that are much more configurable. I'd keep looking.



Much better, more configurable plugins out there to do this

Create great looking Maintenance Mode Page or Coming Soon Page that sets up in minutes.

Dashboard Maintenance Mode vs Rocket Maintenance Mode & Coming Soon Page