Zenoss is an Open Source tool (http://community.zenoss.org/index.jspa) that provides Enterprise Server Management Services.
It supports the modelling and monitoring of applications, networks, servers, and more.
Through SNMP and Syslog access to Enterprise Servers various aspects of servers can be monitored such as CPU usage, memory usage, etc. Thresholds can be set and monitored allowing proactive incident avoidance. It supports the generating of alarms and the sending of emails to notification groups for virtual 24x7 operations support.