Zabbix is an enterprise-level, open source (it's licensed under the GPL v. 2) monitoring software for both networks and applications. One Zabbix server can monitor hundreds of devices, and Zabbix can automatically discover servers and network devices. It is capable of monitoring services without installing any software on the monitored server, but you can also install agents on hosts that will collect data such as CPU load, memory, network, and disk utilization on behalf of the Zabbix server.
pfSense has packages for the Zabbix agent and Zabbix proxy, so you can install the agent or the proxy or both. The difference between the two is that while a Zabbix agent simply collects data from a local host and sends it to the Zabbix server, ...
