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.
Paessler PRTG - An advanced network monitoring solution to monitor network up/downtime, traffic and usage.
FastNetMon - FastNetMon is a very fast DDoS analyzer with sflow/netflow/mirror support.
Observium - Observium
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.