List of Free API's

Web Apps

https://discord.com/developers/docs/intro
https://api.slack.com

Open Source Projects

https://libraries.io/api

Products and Things

https://api.slack.com
https://yelp.com/developers

Health

https://www.healthcare.gov/developers

Governments & Geography

https://code.gov
https://data.gov/developers/apis
https://data.europa.eu/en
https://rapidapi.com/transloc/api/openapi-1-2/details

Food

https://world.openfoodfacts.org/data
https://github.com/evz/tacofancy-api

Gaming

https://steamcommunity.com/dev
https://developer.riotgames.com

Language

https://evilinsult.com/api
https://funtranslations.com/api

Music

https://developer.spotify.com/documentation/web-api

Security

https://haveibeenpwned.com/API/v2
https://developer.shodan.io

Science & Math

https://api.nasa.gov
https://products.wolframalpha.com/api
https://developer.osf.io

Sports

https://any-api.com/nba_com/docs/API_Description

Movies and Comics

https://developer.marvel.com
https://api.chucknorris.io
https://thesimpsonsquoteapi.glitch.me
https://swapi.tech
https://superheroapi.com
https://www.finalspaceapi.com
https://kitsu.docs.apiary.io
http://pokeapi.com
https://rickandmortyapi.com