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.
M/Monit - Monit is a free open source utility for managing and monitoring, processes, files, directories and filesystems on a UNIX system. Monit conducts automatic maintenance and repair and can execute meaningful causal actions in error situations.
Akamai - Akamai is the leading content delivery network (CDN) services provider for media and software delivery, and cloud security solutions.
Munin - PnP networked resource monitoring tool that can help to answer the what just happened to kill our performance