When we are doing maintenance for our blogs, we always want to let visitors know about it. So that they will not be scared or get confused. Maintenance Mode Plugin is a really great tool to help us. Maintenance Mode plugin adds a splash page to your blog that lets visitors know your blog is down for maintenance. This plugin adds the capability “access_on_maintenance” to the role “Administrator”. When you activate the maintenance mode, logged in administrators get full access to the blog including the front-end. When the maintenance mode is activated, the following message is being displayed:

Pricing: Free
Requirements: Wordpress 2.0.9+
Source: http://sw-guide.de/wordpress/plugins/maintenance-mode/