What are some alternatives?
When comparing eZ Server Monitor and Nagios, you can also consider the following products
Zabbix - Track, record, alert and visualize performance and availability of IT resources
PhpSysInfo - phpSysInfo is a customizable PHP script that displays information about your systems.
Datadog - See metrics from all of your apps, tools & services in one place with Datadog's cloud monitoring as a service solution. Try it for free.
RunDeck - RunDeck is an open source automation service with a web console, command line tools and a WebAPI.
Icinga - Icinga is a fork of Nagios and is backward compatible.
Monitor in a Box - Instantly usable, open source monitoring