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.
Mocklets - A HTTP-based API simulator to assist parallel development 🛠️
Screenster - 10x faster UI testing without a line of code.
API List - A collective list of APIs. Build something.
Robot framework - Robot Framework is a generic test automation framework for acceptance testing and acceptance...
Airbnb Official API - Develop your application for millions of travelers & hosts