nfsen-ng - nfsen-ng aims to be an in-place replacement for the ageing nfsen.
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...
FastNetMon - FastNetMon is a very fast DDoS analyzer with sflow/netflow/mirror support.
Graylog - Graylog is an open source log management platform for collecting, indexing, and analyzing both structured and unstructured data.