Postman - The Collaboration Platform for API Development
Apiary - Collaborative design, instant API mock, generated documentation, integrated code samples, debugging and automated testing
Cdyne - Cdyne is a professional REST and SOAP API provider.
Django REST framework - Django REST framework is a toolkit for building web APIs.
LinkedIn - LinkedIn is a business-oriented social networking service, mainly used for professional networking.
Gravitee.io - Gravitee.io is a flexible, lightweight and an open source API management solution.