Are My Sites Up is a free service that monitors Web sites to see if everything is up and running. You simply plug in the URLs of whatever sites you want to keep an eye on (they don't even have to be yours), and it will send you an alert if it notices one is down.
It does this by visiting each page on your list--up to 50 per user--every 15 minutes. If it can't load the page, you get an e-mail. You can also plug in your mobile phone number to get an SMS alert.
I very much like the idea, since you just set it and forget it. It's a nice complement to downforeveryoneorjustme.com which does the same check, but for the people unsuccessfully attempting to visit your site who believe their local undersea cable may have been cut again.