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
TestComplete - TestComplete Desktop, Web, and Mobile helps you create repeatable and accurate automated tests across multiple devices, platforms, and environments easily and quickly.
WebLOAD - WebLOAD - The most flexible and cost effective software for enterprise load, stress and performance testing, integrated with DevOps processes. Click for details
Katalon - Built on the top of Selenium and Appium, Katalon Studio is a free and powerful automated testing tool for web testing, mobile testing, and API testing.
StresStimulus - Load testing tool for websites and mobile that works with hard-to-test applications.