Apache JMeter - Apache JMeter™.
flood.io - Load Testing for Everyone
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.
Loader.io - Loader.io is a simple cloud-based load testing service
locust - An open source load testing tool written in Python.
gatling - A high performance web server. Small! (125k static Linux-x86 binary with HTTP, FTP and SMB support).