Founded in 2001 and headquartered in Riga, Latvia, Zabbix is an open-source enterprise monitoring platform deployed by 10000+ enterprises. A single server manages 10000+ devices with distributed Proxy architecture.
Prometheus, started by SoundCloud in 2012, is a CNCF-graduated open-source monitoring and alerting system that uses a Pull model with built-in PromQL - the de-facto monitoring standard for cloud-native and Kubernetes environments.
Grafana, created by Grafana Labs in 2014 (Stockholm/New York), is the world's most popular open-source monitoring visualization platform with 20M+ users, 100+ data sources and 30+ panel types.
Founded in 1999 and headquartered in Saint Paul, Minnesota, USA, Nagios is the pioneer of IT infrastructure monitoring. Nagios Core is free and open source, while Nagios XI offers web management and reporting.