Realtime and historical market data on all US equities and ETFs.
Alpaca API is a free Finance API that requires apiKey authentication, is served over HTTPS and supports CORS, so it can be called from the browser.
Gain seamless access to a wealth of data with Alpaca Market Data API, offering real-time and historical information for equities, options, crypto and more.
Details from docs.alpaca.markets.

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The Alpaca API is provided by Alpaca US (docs.alpaca.markets). Gain seamless access to a wealth of data with Alpaca Market Data API, offering real-time and historical information for equities, options, crypto and more.
Yes. The Alpaca 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 Alpaca API uses apiKey authentication, so you need to register for credentials before making requests.
Yes. The Alpaca API supports CORS, so front-end JavaScript can call it directly.
Yes. The Alpaca API is served over HTTPS, so it's safe to use in production.