Zabbix - Track, record, alert and visualize performance and availability of IT resources
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.
Icinga - Icinga is a fork of Nagios and is backward compatible.
systemd - systemd is a replacement for the init daemon for Linux (either System V or BSD-style).
Spiceworks Help Desk - Help desk software features that work for IT