Vigil is a simple iPhone app that monitors your website. It's a $9.99 In App Purchase and includes an unlimited number of websites. It let me know a few minutes faster when one of the work websites I manage went down on Sunday night. It includes customizable settings for notification sensitivity that can be managed on a site by site basis.
My only complaint is the IAP model. I will be paying for this through my work, so it's easier to put it directly on a corporate card instead of having to get reimbursed after the fact.