Colorado State Government Open Data.
Colorado Information Marketplace API is a free Government API that requires no API key, is served over HTTPS and has unconfirmed CORS support.

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Yes. The Colorado Information Marketplace 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 Colorado Information Marketplace API requires no authentication, so you can start making requests without signing up.
CORS support for the Colorado Information Marketplace API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The Colorado Information Marketplace API is served over HTTPS, so it's safe to use in production.