Apache JMeter - Apache JMeter™.
JavaMelody - JavaMelody is a powerful full tool that is used to monitor Java and Java EE application servers in production and QA environment.
Blackfire.io - Blackfire empowers all PHP developers and IT/Ops to continuously verify and improve their app’s performance, throughout its lifecycle, by getting the right information at the right moment.
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.
Sitespeed - Sitespeed.io is an open source tool that helps to analyze and optimize website speed and performance.
Grafana - Data visualization & Monitoring with support for Graphite, InfluxDB, Prometheus, Elasticsearch and many more databases