One of my favorites is the cat API. If you're just playing around with APIs and you're not building, well, I don't know why you need this paid version, but it's free. One Graph is a really cool tool that allows you to bring together a single GraphQL endpoint for all your SaaS apps. And then Django rest frameworks, another one that I used to use all the time. But those are just some things to Google if you want to build your own API.