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