Yes, we have two monitoring systems in place that alert us to server problems. One is "on network" (within the same data center) and is setup to monitor bandwidth, CPU load, hard drive space, etc. The second is "off network" (outside the data center) and is configured to monitor access to server services like web, mail, and FTP.
Our on-network monitoring polls servers every minute while our off-network monitoring checks services at 15 minute intervals.
Both systems are configured to notify Jub Jub personnel immediately of any downtime.