Prometheus - An open-source systems monitoring and alerting toolkit.
OpsDash - All-in-one solution for monitoring servers, services, uptime and databases. Curated Dashboards. Rule-based alerting.
Nagios - Complete monitoring and alerting for servers, switches, applications, and services
NIXStats - SaaS monitoring tool for window/linux servers and websites. Think up-time robot but with more features.
StatsD - Simple daemon for easy stats aggregation.
Monitornator - Server monitoring made simple