Via Chris Pirillo, we learn of Monastic "the free website monitoring service that doesn't suck". They check your site every couple of minutes and if it isn't available, they send you an e-mail and notify and RSS feed.
They should add SMS-message and charge as a premium service. Why? Well, (a) your cell phone is probably where you want to get the message and (b) when people/companies make money, the service gets longevity!