matsuoshi

Maintenance Mode for Non-administrator login users. 
This plugin block access to Admin-page by Editor, Writer etc. 
Only Administrator can access the Admin-page while this plugin is active.
Meanwhile, this plugin doesn’t block visitors’ access to the site.

Admin Hot Maintenance Mode

Maintenance Mode for Non-administrator login users. Only Administrator can access the Admin-page.

wpsimpletools

Puts the site in maintenance mode, redirects all non-authenticated requests to a basic responsive page (included in the plugin) or custom URL. 
Maintenance mode setting page is accessible from WP ‘Settings’ menu.

Maintenance Mode with 'coming soon' page or redirect

Simple yet complete maintenance mode plugin.

Admin Hot Maintenance Mode vs Maintenance Mode with 'coming soon' page or redirect