DreamFactory - Turns any SQL/NoSQL/Structured data into Restful API. (Source Code under Apache-2.0 written in PHP)
Fusio - Open-source API management platform which helps to build and manage REST APIs. (Source Code under AGPL-3.0 written in PHP)
Hapttic - Simple HTTP server that forwards all requests to a shell script to handle webhooks you receive. (coded under Apache-2.0 written in Go)
Hasura - Fast, instant realtime GraphQL APIs on Postgres with fine grained access control, also trigger webhooks on database events. (Source Code under Apache-2.0 written in Haskell)
Hoppscotch - A free, fast and beautiful API request builder. (Source Code under MIT written in Nodejs/Vue/Nuxt)
Kong - The World’s Most Popular Open Source Microservice API Gateway and Platform. (Source Code under Apache-2.0 written in Lua)
KrakenD - Open source High-Performance API Gateway. (Source Code under Apache-2.0 written in Go)
Para - Flexible and modular backend framework/server for object persistence, API development and authentication. (Source Code under Apache-2.0 written in Java)
Pizzly - Open-source API Integrations Manager that provides everything a developer needs to interact with OAuth based APIs. (code under MIT written with Nodejs)
Tyk - Fast and scalable open source API Gateway. Out of the box, Tyk offers an API Management Platform with an API Gateway, API Analytics, Developer Portal and API Management Dashboard. (Source Code under MPL-2.0 written in Go)