Nagios provides enterprise-class open source monitoring including data gathering, data archiving, dashboards, and alerting for systems, networks, and infrastructure. Its products include Nagios XI for server and network monitoring, Nagios Log Server for centralized log management, Nagios Network Analyzer for netflow analysis, and Nagios Fusion for a centralized view of monitoring infrastructure.

Originally released in 2002, Nagios is one of the older monitoring tools still available today. Its strengths lie in its scalability for simple and static setups, easy to understand plugin architecture, and deep ecosystem of plugins for extending functionality. It also has a number of popular direct alternatives including Check_MK, which began life as a Nagios extension, and Icinga, a Nagios fork.