Postman - The Collaboration Platform for API Development
MuleSoft Anypoint Platform - Anypoint Platform is a unified, highly productive, hybrid integration platform that creates an application network of apps, data and devices with API-led connectivity.
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.
Azure API Management - Microsoft Azure API Management enables user to create consistent and modern API gateways for existing back-end services hosted anywhere, secure and protect them from abuse and overuse, and get insights into usage and health.
Gravitee.io - Gravitee.io is a flexible, lightweight and an open source API management solution.