Zabbix - Track, record, alert and visualize performance and availability of IT resources
Prometheus - An open-source systems monitoring and alerting toolkit.
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.
Fluent Bit - Data collector and log forwarder.
Grafana - Data visualization & Monitoring with support for Graphite, InfluxDB, Prometheus, Elasticsearch and many more databases
collectd - System and applications metrics collector