Unless you are sat in front of your website 24 hours a day, how will you know if the site goes down?  Wouldn't it be nice to receive an email or a text if for instance Joomla or Moodle stops responding?  I happened to stumble upon the website below:

The free plan will check up to five of your sites at least 25 times per day and you get unlimited email and SMS notifications.  The will also try and tell you WHY your site was down by providing the HTML status error code.  The premium service offers more options and a larger number of sites that can be monitored.

On the face of it it seems like a reasonable service when you consider more complicated options like a server hosting a Nagios or Zenoss installation.