Scrimba:
Scrimba provides interactive coding lessons, including a course on React. You can code inside your browser.
Codecademy:
Codecademy's interactive learning environment will help you get started with React, and practice it.
freeCodeCamp:
freeCodeCamp offers an interactive course on Front End Libraries which includes React.
Frontend Mentor:
Frontend Mentor offers projects you can build. It doesn't provide tutorials, but it's a great way to practice by building real-world projects.
Frontend Mentor | Front-end coding challenges using a real-life workflow
LeetCode:
Although LeetCode is known for data structure and algorithm problems, it has a "Mock" section where you can practice building front-end projects, including ones with React.
LeetCode - The World's Leading Online Programming Learning Platform
CodeSandbox:
CodeSandbox is an online editor that helps you create web applications, from prototype to deployment. It's a great place to practice your React skills.
CodePen:
CodePen is another online code editor where you can write React code directly in your browser.
HackerRank:
HackerRank's "10 Days of React" will give you some good practice problems.
Edabit:
Edabit has a section for React in their web development challenges.
Glitch:
Glitch is a collaborative coding site where you can write React code and see results in real-time.
Glitch: The friendly community where everyone builds the web