What is Zenoss?
The Zenoss project provides an integrated, easy-to-deploy and cost-effective opensource alternative for enterprise IT infrastructure monitoring. Unlike the big OSS companies (IBM, HP, CA, BMC, Amdocs) we are able to keep costs down and directly involve end users throughout the world. More details can be found here.
ZABBIX offers advanced monitoring, alerting and visualisation features today which are missing in other monitoring systems, even some of the best commercial ones.