Useful APIs

https://jsonplaceholder.typicode.com/ (Free REST API to generate fake data)
https://opentdb.com/ (Trivia data API)
https://develop.battle.net/documentation/hearthstone/game-data-apis (Hearthstone Game Data API)
https://superheroapi.com/ (Super hero data API)
https://sv443.net/jokeapi/v2/ (Joke API)

GitHub List of Public APIs

https://github.com/public-apis/public-apis (extensive list of publically available APIs)