Apache JMeter - Apache JMeter™.
OctoPerf - OctoPerf is a SaaS load testing solution, based on JMeter.
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.
StresStimulus - Load testing tool for websites and mobile that works with hard-to-test applications.
locust - An open source load testing tool written in Python.
SmartMeter.io - Performance Testing with Enterprise-Level Features with a familiar jMeter interface