ISS astronauts, current location, etc.
Open Notify API is a free Science & Math API that requires no API key, is only available over HTTP and does not support CORS, so it should be called from a server.
Details from open-notify.org.

Unified API suite · now live
Geocode an address, validate an email, track a flight or pull stock data with one account, one dashboard and one API key.
Repository and archive for study designs, research materials, data, manuscripts, etc
Facts pertaining to the physical science of Oceanography
Global marine species occurrence and distribution data
Sub-arcsecond planetary positions, natal charts, transits, and Human Design from NASA JPL DE440
The Open Notify API is provided by Open Notify -- API Documentation (open-notify.org).
Yes. The Open Notify API is listed in the Science & Math 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 Open Notify API requires no authentication, so you can start making requests without signing up.
Not directly. The Open Notify API does not allow CORS, so call it from a server, serverless function or proxy instead of front-end JavaScript.
No. The Open Notify API is only available over plain HTTP, which browsers block on HTTPS sites. Call it from a server instead.