Postman - The Collaboration Platform for API Development
Beeceptor - Unblock yourself from API dependencies, and build & integrate with APIs fast. Beeceptor helps you build a mock Rest API in a few seconds.
Django REST framework - Django REST framework is a toolkit for building web APIs.
Webhook.site - Instantly generate a free, unique URL and email address to test, inspect, and automate (with a visual workflow editor and scripts) incoming HTTP requests and emails.
Amazon API Gateway - Create, publish, maintain, monitor, and secure APIs at any scale
MockServer - Easy mocking of any system you integrate with via HTTP or HTTPS.