Webhook Tester : Webhook Tester allows you to easily test webhooks and other types of HTTP requests.
Public APIs : A collective list of free APIs for use in web development.
Alpaca : Given a web API, Generate client libraries in node, php, python, ruby
npoint.io : JSON storage bins with schema validation
Cool QL Cool : CoolQLCool (CQC) is an open source GraphQL server that allows you to turn websites into GraphQL APIs.
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.