Flatris is an open source web game. It's a custom Tetris implementation with a multiplayer spin. It uses React, Redux, Socket.IO and Next.js to make the real-time magic happen. Check out the source code if you're into the geeky stuff.
React Cosmos is a dev tool for building scalable, high-quality user interfaces. It's designed to make it easy for you to test and iterate on React components. It makes debugging less painful, and it enables you to create and publish component libraries.