logstash - logstash is a tool for managing events and logs.
Zabbix - Track, record, alert and visualize performance and availability of IT resources
rsyslog - Rsyslog is an enhanced syslogd supporting, among others, MySQL, PostgreSQL, failover log...
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.
Graylog - Graylog is an open source log management platform for collecting, indexing, and analyzing both structured and unstructured data.
Prometheus - An open-source systems monitoring and alerting toolkit.