Apache JMeter - Apache JMeter™.
Dynatrace Load - Dynatrace Load lets you test the performance of web and mobile applications with the realism of load generation from the cloud.
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.
Loader - Load testing and tracking application
Sitespeed - Sitespeed.io is an open source tool that helps to analyze and optimize website speed and performance.
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.