An API (Application Programming Interface) allows you to send requests to a remote data set, like querying a database. The response will then be delivered back to your own application in a standardized format. This is very helpful when you're building an app or pulling metrics for reporting, because it means you can focus on presenting information in a unique or useful manner, rather than developing the underlying data set. For example, most weather apps get their weather forecast data from a weather API, rather than building weather stations themselves.
Of course, the fastest way to understand how APIs work is to try them out. While most APIs require access via API keys (which are similar to passwords), or have complex methods of authentication and authorization, there are also quite a few APIs with no requirements at all. This is especially useful for beginners, as it means you can start exploring different APIs right away. It is also useful for web developers looking to access a sample data set for their app without any red tape.
Big List of Free Open APIs
The APIs below can be accessed using your web browser, cURL for the command line, API tools like Swagger and Postman, or Mixed Analytics' own API Connector for Google Sheets. Just click the sample URLs in the table to see their response in your browser, or copy/paste the URLs and run in your tool of choice. You can also modify the URLs to retrieve additional information (consult the documentation links for specifics).
All URLs have been tested and are active and available as of February 2020.
Want even more APIs? Check out these directories: