Zabbix - Track, record, alert and visualize performance and availability of IT resources
Paessler PRTG - An advanced network monitoring solution to monitor network up/downtime, traffic and usage.
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.
Observium - Observium
FastNetMon - FastNetMon is a very fast DDoS analyzer with sflow/netflow/mirror support.
ThousandEyes - ThousandEyes is a Network Intelligence platform that delivers visibility into every network your organization relies on.