Never accidentally edit your live site again!
This super simple WordPress plugin hides your WordPress site from people who aren't logged in, displays a simple message for people who aren't logged in (see screengrab), adds a bright red favicon and changes the color of the WP admin bar to bright red to help you know you are editing a DEV site.
Additionally, you can add an image using the WP media uploader and replace the default message with whatever you'd like, including html markup:
All logged-in users will see the site without a "Maintenance Mode" message, only the admin bar and favicon will be red.