Zabbix - Track, record, alert and visualize performance and availability of IT resources
The Dude - MikroTik is a Latvian company which was founded in 1996 to develop routers and wireless ISP systems. MikroTik now provides hardware and software for Internet connectivity in most of the countries around the world.
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.
Pandora FMS - Pandora FMS: Opensource monitoring software
Prometheus - An open-source systems monitoring and alerting toolkit.
cStatus - Monitor and scan a network.