Yahoo Finance
Real time low latency Yahoo Finance API for stock market, crypto currencies, and currency exchange
Budgeting & Planning.
YNAB API is a free Finance API that requires OAuth authentication, is served over HTTPS and supports CORS, so it can be called from the browser.
Details from api.youneedabudget.com.

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Real time low latency Yahoo Finance API for stock market, crypto currencies, and currency exchange
US market data from SEC, FINRA, CFTC and the Fed: filings, insiders, earnings dates, short interest
The YNAB API is provided by YNAB API (api.youneedabudget.com).
Yes. The YNAB API is listed in the Finance 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.
Yes. The YNAB API uses OAuth authentication, so you need to register for credentials before making requests.
Yes. The YNAB API supports CORS, so front-end JavaScript can call it directly.
Yes. The YNAB API is served over HTTPS, so it's safe to use in production.