I’ve been using the $20/year Vigil iOS app to monitor my Web sites. It doesn’t offer a lot of options or information, but it’s easy to set up and can notify you via iOS push notifications.
Neil Brewitt told me about Uptime Robot, which is free and seems to offer a lot more: an API, notifications via e-mail, SMS, RSS, etc. To receive iOS push notification, you need the free Boxcar app (or, I guess, one of the unofficial clients). Or, if you follow @uptimerobot on Twitter, it can send direct messages to your Twitter client of choice.
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