Postman

Postman is a comprehensive, cross-platform application that provides developers with a set of powerful yet fairly accessible tools for sharing, testing, documenting and monitoring APIs, basically covering the full spectrum of API development.
Originally available only as a Google Chrome extension, you should know that this utility can now be deployed and run just as efficiently on all major OSes out there, thanks to its smooth-running and easy to install Windows, macOS, and Linux clients.
This said, getting started with Postman requires you to undergo a hassle-free installation process, followed by a quick log in procedure. You can use your official Postman credentials, or you can create an account in practically no time using an existing Google account as a starting point.

User-friendly and very well-documentation API development tool

The utility boasts an intuitive and user-friendly interface with support for two themes, one more suited for very well lit work environments while the other for darker ones.
Even though the layout is quite intuitive, we highly recommend that you check out the documentation section provided on the developer's website, just to speed up the learning process a bit.

Advanced tools for testing and managing APIs, wrapped around an easy-to-use interface

Constructing requests and reading responses, track changes within APIs, and take advantage of a multitude of collections and environments for testing and sharing APIs.
If you go for the paid versions, dubbed Postman Pro and Postman Enterprise, besides all the more advanced monitoring security and administration-orientated tools, you also get some neat collaboration features, so you and your team can develop API within a unified and distraction-free environment.

All aspects of API development made a lot easier with the help of Postman

With all that in mind, it is no wonder that Postman has gained so much popularity since it was first launched as a Chrome add-on as it makes the process of thoroughly testing, documenting and sharing APIs seem almost seamless.
As an ending note, Postman is definitely a must-have (or at least a must-try) for every developer that regularly works with APIs for their web or mobile-based projects.