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
Sauce Labs - Test mobile or web apps instantly across 700+ browser/OS/device platform combinations - without infrastructure setup.
WebLOAD - WebLOAD - The most flexible and cost effective software for enterprise load, stress and performance testing, integrated with DevOps processes. Click for details
Browsershots - Browsershots makes screenshots of your web design in different browsers.
StresStimulus - Load testing tool for websites and mobile that works with hard-to-test applications.