RRDTool - High performance data logging and graphing system for time series data
Zabbix - Track, record, alert and visualize performance and availability of IT resources
MRTG - MRTG (or Multi Router Traffic Grapher) is an open-source network monitoring tool.
Abusix - Abusix is a network security company that offers email threat protection and network abuse handling solutions to ISPs, Telcos, and Cloud/Hosting Providers to stop the rapidly growing problem of email-borne threats and other kinds of network abuse.
Ganglia - Scalable distributed monitoring system
FastNetMon - FastNetMon is a very fast DDoS analyzer with sflow/netflow/mirror support.