CloudFlare - Cloudflare is a global network designed to make everything you connect to the Internet secure, private, fast, and reliable.
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.
Datadog - See metrics from all of your apps, tools & services in one place with Datadog's cloud monitoring as a service solution. Try it for free.
MRTG - MRTG (or Multi Router Traffic Grapher) is an open-source network monitoring tool.
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.