Pingit

PingIt is a handy mobile app to get alerted when your website is down before your users find it and shout out.

You can monitor three web sites from the free version.
The app alerts by creating a notification on your phone, and/or by sending an SMS to phone that you can configure.
You can configure your test frequency as 5 minutes, 10 minutes or any multiple of 5 minutes.
In order to relieve you from the repeat alerts, you can set a "backoff" to avoid notification on the same event for "n" cycles.

Disclaimer : Due to network conditions, occasionally a response from the server might not reach your device in time, leading to false positive ( declaring the server as down).