Postman - The Collaboration Platform for API Development
Apiary - Collaborative design, instant API mock, generated documentation, integrated code samples, debugging and automated testing
Django REST framework - Django REST framework is a toolkit for building web APIs.
Paw.cloud - Paw is a REST client for Mac.
Amazon API Gateway - Create, publish, maintain, monitor, and secure APIs at any scale
Gravitee.io - Gravitee.io is a flexible, lightweight and an open source API management solution.