Prometheus - An open-source systems monitoring and alerting toolkit.
M/Monit - Monit is a free open source utility for managing and monitoring, processes, files, directories and filesystems on a UNIX system. Monit conducts automatic maintenance and repair and can execute meaningful causal actions in error situations.
Nagios - Complete monitoring and alerting for servers, switches, applications, and services
Zabbix - Track, record, alert and visualize performance and availability of IT resources
StatsD - Simple daemon for easy stats aggregation.
Hosted Graphite - Graphite as a service