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.
WebLOAD - WebLOAD - The most flexible and cost effective software for enterprise load, stress and performance testing, integrated with DevOps processes. Click for details
locust - An open source load testing tool written in Python.
StresStimulus - Load testing tool for websites and mobile that works with hard-to-test applications.