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.
Postman - The Collaboration Platform for API Development
TestComplete - TestComplete Desktop, Web, and Mobile helps you create repeatable and accurate automated tests across multiple devices, platforms, and environments easily and quickly.
Sauce Labs - Test mobile or web apps instantly across 700+ browser/OS/device platform combinations - without infrastructure setup.
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.
Cypress.io - Slow, difficult and unreliable testing for anything that runs in a browser. Install Cypress in seconds and take the pain out of front-end testing.