I just had to write about this plugin because I love it and have used it more then once. It is the Maintenance Mode plugin.
When making changes to your site you may not want your visitors to see your site in it’s down condition. That’s where this plugin comes in.
The maintenance mode plugin adds a splash page to your site informing visitors to check back. Visitors will see a message like “Maintenance Mode – SITE is currently undergoing scheduled maintenance. Please try back in 60 minutes. Sorry for the inconvenience.”
You simply activate the plugin when you are working on your site and then deactivate it when you are done. Make sure you don’t forget to deactivate it (like I have in the past J if you do forget your visitors will see nothing but the splash page until you do.
The maintenance mode has come in very handy for me when working on blog issues or changes. The plugin and an explanation can be found here.