One of the most common questions we get from open source users is what's the difference between Zenoss and Nagios. While Nagios has one of the largest installed bases of any monitoring solution it's a logical question but Zenoss has a number of features that differentiate it from Nagios. Nagios is basically a scheduler that executes service checks and tests against network devices and reports back the results. They also have a comprehensive list of extensions or plugins that are similiar to Zenoss plugins. So that you can still use the plugins you used with Nagios, Zenoss provides the ability to execute these plugins.

Automatic Discovery - Zenoss can automatically discover hosts and start monitoring them automatically. Nagios requires that you manually enter them in a configuration file.
Performance Monitoring - Zenoss can give you time series graphs on performance. Nagios can't without additional plugins or software. Often users use the popular graphing engine, Cacti, which leverages RRDTool to gather time-series data. Zenoss is the gold sponsor of RRDTool.
Event Management - Zenoss can normalize data into events, beyond collection from other source and de-dupe the events storing occurrences in a counter which than allows users to set alerts based on the frequency of an alert.
Syslog Monitoring - Zenoss can monitor system logs from Unix, Linux, Windows and more. In addition Zenoss can then be used to translate that data into events and normalized along with availability and performance data gathered via other methods like SNMP and SSH.
Sophisticated Alerting - Nagios alerts can be sent to an email address or a pager based on certain conditions being met. However, Zenoss has a much more robust alerting system were alerts can be sent to a single individual or a group of individuals. Escalation rules can be enforced so that when alerts go without acknowledgement, they can be promoted to another network operator.