I have a backlog of React submissions, so I thought I'd work through them rather than spacing them out over a few weeks. First, React Component Playground (GitHub: FormidableLabs/component-playground, License: MIT, npm: component-playground) by Ken Wheeler is a React component that allows you to edit a React component with a live preview. You can try it out on the project's homepage -- see the button example near the top.
Normally with React/Angular/etc. you're constantly waiting for the build/refresh cycle, but with this project you can experiment a little bit to get results more quickly. As the example suggests, this is useful when working with React styles.