Data.gov
Data.gov is a public site that allows for anyone to freely access JSON, XML, and CSV files that contain public information. This information is based on studies, census, and surveys conducted by the federal government, states, and cities.
catalog.data.gov
Socrata
Socrata has a large library of APIs that can be publicly used for free. This site is also able to help visualize that data and it can be saved as a JSON file to implement into you're own code. There are a multitude of different datasets that can be found on this site.
dev.socrata.com
Link to where a dataset can be found.
ProPublica
ProPublica is a investigative journalist outlet that has datasets and APIs that can be used. Some are locked behind a paywall and others are free for anyone to use. The datasets that require payment can be anywhere from $200 to $10,000. You purchase a licnese from ProPublica to use their dataset and then have access to the data and API call from that point on.
propublica.org