Apache JMeter - Apache JMeter™.
Loader.io - Loader.io is a simple cloud-based load testing service
Selenium - Selenium automates browsers. That's it! What you do with that power is entirely up to you. Primarily, it is for automating web applications for testing purposes, but is certainly not limited to just that.
LoadFocus - Cloud Testing Infrastructure | Cloud Testing Services and Tools for Websites & APIs.
locust - An open source load testing tool written in Python.
Loadium - Performance Testing & Load Testing Tool