GTmetrix - GTmetrix is a free tool that analyzes your page's speed performance. Using PageSpeed and YSlow, GTmetrix generates scores for your pages and offers actionable recommendations on how to fix them.
Zabbix - Track, record, alert and visualize performance and availability of IT resources
PageSpeed Insights - PageSpeed is addon for ...
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.
NewRelic - New Relic is a Software Analytics company that makes sense of billions of metrics across millions of apps. We help the people who build modern software understand the stories their data is trying to tell them.
Nagios - Complete monitoring and alerting for servers, switches, applications, and services