I've got a few sites monitored with Hyperspin - nice little service and you can choose the frequency at which your site is monitored. It's save me a few pounds with AdWords the last year I've been using it.
They will contact you by MSN, email, text or even phone depending how severe the outage is. The MSN's are good - you can set it to paranoia level, and not worry too much about getting pestered by loads of spam. They also do a nice free trial, and they're simple to use and friendly.