Zabbix is an excellent open source network monitoring tool. It has been very helpful for the monitoring of our devices which includes not only our network device but even AV and Printers as well.

Good out of box monitoring capabilities with relatively straight forward configuration for a multitude of devices and platforms

Ease of use. Really easy to navigate across different options. Easy to set up. Works with any type of server. Agent consumes very less resources on the running server.