David Herrera

Checks whether the Maintenance Mode of the <a href="https://wordpress.org/extend/plugins/wp-maintenance-mode/" rel="ugc">WP Maintenance Mode</a> 
plugin is on and, if so, adds an alert to that effect in your Admin 
Bar.
Note that WP Maintenance Mode includes an admin bar alert as of 
version 1.6.0, making this plugin redundant for those who use that 
version or above.

WP Maintenance Mode Admin Bar Alert

Checks whether the Maintenance Mode of the WP Maintenance Mode plugin is on and, if so, adds an alert to that effect in your Admin Bar.

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.

WP Maintenance Mode Admin Bar Alert vs Rocket Maintenance Mode & Coming Soon Page