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.
