pfSense - pfSense is a free and open source firewall and router that also features unified threat management, load balancing, multi WAN, and more
Zabbix - Track, record, alert and visualize performance and availability of IT resources
OPNsense - OPNsense® you next open source firewall. Free Download. High-end Security Made Easy™. Offers Intrusion Prevention, Captive Portal, Traffic Shaping and more.
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.
IPFire - IPFire is a server distribution with intended to use as a firewall.
Prometheus - An open-source systems monitoring and alerting toolkit.