Mockoon : Mockoon is the easiest and quickest way to design and run mock APIs. No remote deployment, no account required, free and open-source.
pump.io : Social server with an ActivityStreams API. It's a stream server that does most of what people really want from a social network.
Webhook Tester : Webhook Tester allows you to easily test webhooks and other types of HTTP requests.
Cool QL Cool : CoolQLCool (CQC) is an open source GraphQL server that allows you to turn websites into GraphQL APIs.
htty : htty is a console application for interacting with web servers. It’s a fun way to explore web APIs and to learn the ins and outs of HTTP.
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.