SurviveJS - Webpack and React - project oriented free ebook about ReactJS and Webpack.
ReactJs Fundamentals and the whole Egghead ReactJs Section - some of the content is paid, but the quality of the materials are pure gold.
ReactJS Newsletter - weekly newsletter of the best React.js news, articles, projects, and more.
Medium's ReactJS section - blog posts about everything related to ReactJS and its ecosystem.
React JS Tutorials by LearnCode.academy - from Webpack configuration, through React basics, to Flux and Redux.
Codementor's ReactJS Resources - tutorials, videos, tips and tricks, courses, etc.
React.js Conf 2016 talks - If you missed it, you should definitely watch the videos.