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.
OpsDash - All-in-one solution for monitoring servers, services, uptime and databases. Curated Dashboards. Rule-based alerting.
NewRelic - New Relic is a Software Analytics company that makes sense of billions of metrics across millions of apps. We help the people who build modern software understand the stories their data is trying to tell them.
NIXStats - SaaS monitoring tool for window/linux servers and websites. Think up-time robot but with more features.
Zabbix - Track, record, alert and visualize performance and availability of IT resources
Monitornator - Server monitoring made simple