Data.gov
US Government Data
Contains live datasets including information about petitions, bills, MP votes, attendance and more.
Data.parliament.uk API is a free Government API that requires no API key, is only available over HTTP and has unconfirmed CORS support.
A list of open datasets supported by the Parliamentary Digital Service
Details from explore.data.parliament.uk.

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US Government Data
This API provides read access to DIP entities (e.g. activities, persons, printed material)
Contains D.C. government public datasets, including crime, GIS, financial data, and so on
Yes. The Data.parliament.uk API is listed in the Government category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
No. The Data.parliament.uk API requires no authentication, so you can start making requests without signing up.
CORS support for the Data.parliament.uk API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
No. The Data.parliament.uk API is only available over plain HTTP, which browsers block on HTTPS sites. Call it from a server instead.