Mapstraction : A javascript library to hide differences between mapping APIs.
Alpaca : Given a web API, Generate client libraries in node, php, python, ruby
Strapi : Open source Node.js Headless CMS to easily build customisable APIs
httpbin.org : A simple HTTP Request & Response Service.
FastAPI : FastAPI is a modern, fast (high-performance), web framework for building APIs with Python 3.6+ based on standard Python type hints.
Bluepine is a DSL for defining API Schema/Endpoint with the capabilities to generate Open API (v3) spec, validate API request and serialize object for API response based on single schema definition.