Recommendation for notification service when web server is down [closed]

Can anyone recommend a good third party service that can notify you when your webserver is down?

I signed up for a Pingdom account a few days ago for a new site, and have found it to be really good. I signed up for their "Basic" Account, which includes 5 checks, 20 SMS alerts (could be higher) and unlimited emails for $9.95/month. I haven't been able to test how effective it is at alerting me when the server is down, as I have luckily not had any downtime on my site.

Here is simple service:

Down Notifier http://downnotifier.appspot.com/

1) it's free! 2) it's easy to start (10 seconds to subscribe to notification) 3) it's really easy! no need to register.

AlertFox offers web monitoring plus web transaction monitoring. It runs the tests in real IE and Firefoxm, and you receive a screenshot of the site when an error is detected - very useful. It proved useful to convince our sys admin that there was indeed an error on "his" server (php update bug) :D