Random User Generator API
The Random User Generator API is a free and publically available API provided by randomuser.me that specializes in generating fake user data for website testing purposes.
The website generates example javascript and displays the result of the javascript beneath. The javascript can be edited directly by the user to test changes and see in real time how those changes are reflected in the result.
Random User Generator is incredibly easy to use and provides as both an excellent tool for testing javascript, but also serves as a helpful learning tool in regard to javascript.
Ball Don't Lie API
Balldontlie API is a free and publically available API provided by balldontlie.io. This API provides real NBA data from the years 1979 - now, covering: players, teams, games, stats, and averages.
An advantage of using this website is that the data can be compared to true data, helping to reinforce if code is functional or not. With such a great compilation of data, more dynamic code can be tested, such as code that would interface with a database.
Balldontlie.com is a wonderful go to when looking to test code that may need to sift through a wide array of data. This true data can be compared with code results to gauge if the functionality is accurate or in need of work.
The Cat API
Thecatapi is a free and publically available API provided by thecatapi.com. This API functions more similarly to Ball Don't Lie than Random User Generator as it's used to access a wide array of information, in this case, all about cats.
A unique aspect of this API is that it incorporates pictures and the ability to search based on a chosen uploaded picture. It seems that the creators of the cat API also offer multiple other API's.
Summary
API's can be utilized to provide a wide array of data, or even media. There are many websites built simply to provide as test servers.
The internet is, in a sense, a large API and the websites devoted to helping web developers build websites are assisting in the modernization of the web as a whole.