Postman is an API platform for building and using APIs. Postman simplifies each step of the API lifecycle and streamlines collaboration so you can create better APIs—faster.
It’s an API repository. You can easily store, catalog, and collaborate around all your API artifacts on one central platform. Postman can store and manage API specifications, documentation, workflow recipes, test cases and results, metrics, and everything else related to APIs.
The Postman platform includes a comprehensive set of tools that help accelerate the API lifecycle—from design, testing, documentation, and mocking to the sharing and discoverability of your APIs.
Postman workspaces help you organize your API work and collaborate across your organization or across the world. There are four different kinds of Postman workspaces for your different needs: personal, team, partner, and public workspaces.
Postman integrates with the most important tools in your software development pipeline to enable API-first practices. The Postman platform is also extensible through the Postman API and through open source technologies.
You can use the Microsoft Graph Postman collection to get started with Microsoft Graph APIs in minutes.
Free, available on the web, it provides a platform to test API references with additional functionality to Microsoft Graph API, especially if you are referencing multiple APIs in your application. There is an App (Client Version) you can install if you find you are using it for a lot of things, or you can just do it all in your browser. Depends on your preferences, as with most things.
You can visit Postman here