Prometheus - An open-source systems monitoring and alerting toolkit.
logstash - logstash is a tool for managing events and logs.
Nagios - Complete monitoring and alerting for servers, switches, applications, and services
rsyslog - Rsyslog is an enhanced syslogd supporting, among others, MySQL, PostgreSQL, failover log...
StatsD - Simple daemon for easy stats aggregation.
Graylog - Graylog is an open source log management platform for collecting, indexing, and analyzing both structured and unstructured data.