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
TestRail - TestRail provides comprehensive test case management for software testing. Organize your testing, boost productivity, get real-time insights, and track progress toward milestones. Integrates with leading issue tracking and test automation tools.
gatling - A high performance web server. Small! (125k static Linux-x86 binary with HTTP, FTP and SMB support).