There are several sites that have information about popular APIs, like Apilist https://apilist.fun/ and RapidAPI https://rapidapi.com/collection/list-of-free-apis

Here are a few well documented APIs:

* For communications, Twilio API offers lots of flexibility and scale https://www.twilio.com/docs
* Last.FM offers quite a strong API for Music projects https://www.last.fm/api
* For mapping, you have the OpenStreetMap project has quite a lof of data https://wiki.openstreetmap.org/wiki/Databases_and_data_access_APIs
* Open Meteo offers a straight forward API for current weather and forecasts https://open-meteo.com/en/docs
* Stripe API is well regarded in financial transactions https://stripe.com/docs/api
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