Want to use federal data? The U.S. government used to have a prominent link to sign up for API access; that link has been removed, but the form to get it remains operative. You may wish to sign up now, on the premise you might not be able to later. https://api.data.gov/signup/
Comments
https://api.data.gov/docs/developer-manual/
Agency manual
https://api.data.gov/docs/agency-manual/
Agency manual lists examples of Swagger docs for APIs using https://api.data.gov.
An entry point to the docs: https://docs.ckan.org/en/2.11/api/index.html#get-able-api-functions
A sample query: https://catalog.data.gov/api/3/action/package_search?q=properties
Details I needed https://catalog.data.gov/api/3/action/package_show?id=inventory-of-owned-and-leased-properties-iolp