What are some alternatives?
When comparing Eyes Of Network and Nagios, you can also consider the following products
DiamanteDesk - DiamanteDesk is an open-source easy-to-use help-desk solution.
Zabbix - Track, record, alert and visualize performance and availability of IT resources
openITCOCKPIT - openITCOCKPIT is an open source out of the box monitoring framework. It can be installed on Debian based distributions through apt and provides a user friendly web interface where you can create all your hosts and services you like to monitor.
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.
Spiceworks Help Desk - Help desk software features that work for IT
Icinga - Icinga is a fork of Nagios and is backward compatible.