Apache JMeter - Apache JMeter™.
Optimizely - A/B testing you'll actually use.
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.
VWO - Use VWO to tweak, optimize & personalize your website with minimal IT help.
locust - An open source load testing tool written in Python.
Google Analytics - Improve your website to increase conversions, improve the user experience, and make more money using Google Analytics. Measure, understand and quantify engagement on your site with customized and in-depth reports.