Zabbix - Track, record, alert and visualize performance and availability of IT resources
PagerDuty - Cloud based monitoring service
Observium - Observium
OpsGenie - Alerting and On-Call Management for Dev&Ops Teams
Nagios - Complete monitoring and alerting for servers, switches, applications, and services
xMatters - xMatters transforms event data into intelligent communication IT teams, avoiding outages and disruptions