NotifyMee
A Single Function SiteDescription:
NotifyMee is a handy service that notifies you if a downed site is up and running again. Using the service is simplified in 3 steps; 1) enter the URL of the site you want to be notified about, 2) add to which email you want the notification to be sent, and 3) click on "Thank You!". Once the site is up again, you will receive an email notifying you of the new status, and as Twitter is somehow becoming the source of information, the service will also send a tweet to let other people know about it.
That being said, if you want to know what sites are up after they disappeared from the internet, you can follow the updates on Twitter @notifymee.