Private businesses, public institutions and non-profits sometimes make their data accessible to the public in the form of web APIs. Some hope that independent developers will come up with innovative applications that in-house teams are too busy to experiment with. Some are just committed to the transparent and open nature of the web. Whatever their motivation, each company or institution will have its own policies towards what data is revealed and how it is accessed. Sometimes these policies change which can render previously functioning code obsolete.
If you don't have it already, download the Chrome browser. Then download, a JSON formatter.
Identify a web API that returns JSON data. Sign up for it. Describe the data in a short paragraph and why it's available to the public and submit to this document. You may start with this list, but I encourage you to explore your own ideas. You will be amazed at how many websites offer their data.