Postman - The Collaboration Platform for API Development
Django REST framework - Django REST framework is a toolkit for building web APIs.
Cdyne - Cdyne is a professional REST and SOAP API provider.
Amazon API Gateway - Create, publish, maintain, monitor, and secure APIs at any scale
LinkedIn - LinkedIn is a business-oriented social networking service, mainly used for professional networking.
Apiary - Collaborative design, instant API mock, generated documentation, integrated code samples, debugging and automated testing