The following is an introduction to the official API Client released this time, excluding the API Client that was originally reverse engineered.
Also, as of now (2020/05/16), the README must be reasonably well-developed.

Official API Approach

JavaScript

The only SDK that is officially provided by Notion.

https://github.com/makenotion/notion-sdk-js

Go

https://github.com/dstotijn/go-notion
https://github.com/ketion-so/go-notion
https://github.com/sorcererxw/go-notion
https://github.com/mkfsn/notion-go

Python

https://github.com/ramnes/notion-sdk-py

PHP

https://github.com/5am-code/laravel-notion-api
https://github.com/64robots/php-notion

Ruby

https://github.com/mgmarlow/notion-sdk-ruby

Reverse Engineering Approach

Python and Go are the two most famous API clients that use reverse engineering techniques. Both of them have more than 1,000 stars, and I think many users are using them.

https://github.com/jamalex/notion-py
https://github.com/kjk/notionapi

It is still unclear if this one will support the official API.
I'm not sure if this one will support the official API or not, but it will have to be a major change from the current implementation, and there are many operations that are not supported by the official API, so I don't think it's realistic to follow suit.