Zabbix - Track, record, alert and visualize performance and availability of IT resources
PagerDuty - Cloud based monitoring service
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.
VictorOps - We make on-call suck less & help teams to solve problems faster.
Prometheus - An open-source systems monitoring and alerting toolkit.
Grafana - Data visualization & Monitoring with support for Graphite, InfluxDB, Prometheus, Elasticsearch and many more databases