I have been evaluating opensource software applications for Network and System Monitoring for quite a sometime. As most of the organizations do not authorise budget for monitoring software and the preasure is often put on IT to deal with the uptime SLA's.
I evaluated Nagios, Icinga and Zenoss. Icinga is a fork of nagios and is backward compatible. Zenoss on the other hand has two versions community version for free and Enterprise verion at a cost. Icinga and Zenoss community version has its own pros and cons.
However, Zenoss has a appealing interface with integration to goolgle maps and is equiped with real time graphs. Zenoss also have numerous plugins (zenpacks) that supports different hardware models. Most of all zenoss is agentless !!!. Which makes it a more prudent option in a data centre environment. The application owners wouldn't want any other software agents installed on the servers and agents cannot be installed without the concent from the application vendors. Therefore the agentless option is the way forward. Icinga on the other hand needs a agent to be installed does't provide out of the box performance graphs.