As we all experience, every hosting account or server sometimes fails, no matter how good the hosting company is. And, this can be accepted as normal when it is within the limits.
For better reacting to the downtimes, analyzing the health of the web application & the environment it is hosted on, uptime monitoring is a working solution.
phpWatch is a free PHP uptime monitoring script that can watch unlimited number of websites and send notifications via e-mail or SMS (for US phones).
With an Ajaxed interface, new monitors & notifications can be easily configured. And, once the cron job / scheduled task -that pings the servers in desired time intervals- is set, the script is ready to go.
phpWatch also offers an API which lets other applications to query the monitored services, get the statistics and use the notification system for sending SMS and e-mail alerts.
Requirements: PHP, MySQL