Zabbix - Track, record, alert and visualize performance and availability of IT resources
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.
FastNetMon - FastNetMon is a very fast DDoS analyzer with sflow/netflow/mirror support.
Observium - Observium
Cacti - Cacti is a complete network graphing solution. It provides a fast poller, advanced graph templating, multiple data acquisition methods, and user management features.
Wireshark - Wireshark is a network protocol analyzer for Unix and Windows. It lets you capture and interactively browse the traffic running on a computer network.