Apache JMeter

Apache JMeter is the venerable, open-source standard for performance and load testing, with robust capabilities for functional API testing. Originally designed for web applications, it supports a wide array of protocols (HTTP, HTTPS, SOAP, REST, FTP, JDBC, JMS, etc.) through samplers. Its strength lies in creating highly configurable, complex test plans with logic controllers, pre/post processors, and listeners for detailed analysis. While its UI is considered dated, its power, flexibility, and zero cost are unmatched for performance-centric testing.

It's ideal for performance engineers, developers, and QA professionals who need to conduct heavy load, stress, and spike testing on APIs and backend services, and who are comfortable with a steeper, more technical learning curve for ultimate control.