Prometheus - An open-source systems monitoring and alerting toolkit.
NinjaOne - NinjaOne (Formerly NinjaRMM) provides remote monitoring and management software that combines powerful functionality with a fast, modern UI. Easily remediate IT issues, automate common tasks, and support end-users with powerful IT management tools.
Nagios - Complete monitoring and alerting for servers, switches, applications, and services
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.
StatsD - Simple daemon for easy stats aggregation.
SolarWinds Orion Platform - SolarWinds Orion Platform is an infrastructure monitoring and management solution that has been designed to streamline and manage the composite and physically dispersed IT infrastructure.