Newgrounds.io is an API built for indie game developers. It was originally designed for web-based games (specifically ones made in Adobe Flash), and has evolved over the years into the platform-agnostic version you see today.
Any developer can implement this API, absolutely free*, you just need a Newgrounds.com account, and a game project to get started (see Getting Started page for details).
* Future updates MAY introduce new/advanced features that will require a subscription. Who can tell what the future holds?
Follow the links below to learn more about using Newgrounds.io
Communicating with the Server
Learn how to communicate directly with the server, without using a pre-made library.
Learn when/how/why to use encryption when calling server components.
Learn what components are available, what parameters you need to pass them, and what return data to expect.
Newgrounds.io Object Models
Learn how many of the objects you will be using should be structured.
Learn how Newgrounds.io works with Newgrounds passport to get users logged in.