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
MRTG - MRTG (or Multi Router Traffic Grapher) is an open-source network monitoring tool.
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.
Akamai - Akamai is the leading content delivery network (CDN) services provider for media and software delivery, and cloud security solutions.
Nagios - Complete monitoring and alerting for servers, switches, applications, and services