USAspending.gov
US federal spending data

US congressional stock-trade disclosures and the President's OGE reports, linked to each filing.
Veridion API is a free Government API that requires no API key, is served over HTTPS and supports CORS, so it can be called from the browser.
Reference for the Veridion Disclosure API 1.21.1: authentication, every endpoint and parameter, response fields, pagination, point-in-time reads, incremental sync, export verification, errors, and the stability policy. Rendered from the OpenAPI document.
Details from veridionmarkets.com.

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US federal spending data
Live open UK public sector tenders by trade from Find a Tender and Contracts Finder (OGL v3.0)
Point-in-time amendment history for UK law
The Veridion API is provided by Veridion (veridionmarkets.com). Reference for the Veridion Disclosure API 1.21.1: authentication, every endpoint and parameter, response fields, pagination, point-in-time reads, incremental sync, export verification, errors, and the stability policy. Rendered from the OpenAPI document.
Yes. The Veridion 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 Veridion API requires no authentication, so you can start making requests without signing up.
Yes. The Veridion API supports CORS, so front-end JavaScript can call it directly.
Yes. The Veridion API is served over HTTPS, so it's safe to use in production.