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.
Sauce Labs - Test mobile or web apps instantly across 700+ browser/OS/device platform combinations - without infrastructure setup.
OctoPerf - OctoPerf is a SaaS load testing solution, based on JMeter.
Browsershots - Browsershots makes screenshots of your web design in different browsers.
SmartMeter.io - Performance Testing with Enterprise-Level Features with a familiar jMeter interface