Cacti

Cacti wraps RRDtool in a web interface, so the time-series math and round-robin storage happen under the hood while you work with graphs, data sources, and templates. Point it at network gear, servers, or applications and it polls them on a schedule, keeping a fixed-size history of every metric.

Device discovery finds hosts to monitor, and templating lets one graph definition apply across hundreds of similar devices. SNMP polling covers v1, v2, and v3 with IPv6, and data can also come from scripts or command output. Role-based access control, a plugin framework, remote data collectors for distributed polling, a realtime view, and CSV export round it out.

Cacti runs on a web server with PHP, which doubles as the CLI for scheduled polling and maintenance. It is released under GPL v2 and installed from source.