Scout - Scout ♥ monoliths.
Sitespeed - Sitespeed.io is an open source tool that helps to analyze and optimize website speed and performance.
AppOptics - Application performance management and infrastructure monitoring.
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.
CXGuardian - CXGuardian is a platform powered by QualityKiosk Technologies and allows users to monitor their mobile and web-based applications’ performance.
Apache JMeter - Apache JMeter™.