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
OpsDash - All-in-one solution for monitoring servers, services, uptime and databases. Curated Dashboards. Rule-based alerting.
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.
Monitornator - Server monitoring made simple
Prometheus - An open-source systems monitoring and alerting toolkit.