Thankifi is a free (as in speech) API that makes it easy for you to express gratitude. There is no need to grab any api keys to use it either.
In order to guarantee opeartional status of the API for everyone, you are allowed a maximum of 50 calls per 10 seconds. If your application grows to be constrained by the rate limiting applied to Thankifi, feel free to selfhost your own or contact me to talk about it.
Check out the open api reference for a description of each available endpoint.