1. JSFiddle

https://jsfiddle.net/ Languages: HTML, CSS, JavaScript Why it’s useful: A popular platform for testing and sharing front-end code. It offers live previews and collaboration features.

2. CodePen

https://codepen.io/ Languages: HTML, CSS, JavaScript Why it’s useful: An excellent tool for front-end designers and developers to test and showcase their web development projects.

3. Repl.it

https://replit.com/ Languages: Multiple (Python, JavaScript, Ruby, C++, and more) Why it’s useful: Supports a wide variety of programming languages, providing an instant development environment for building and sharing code.

4. JSBin

https://jsbin.com/ Languages: HTML, CSS, JavaScript Why it’s useful: A quick and easy way to test front-end code with real-time preview and debugging options.

5. Codesandbox

https://codesandbox.io/ Languages: JavaScript, React, Vue, Angular, Node.js Why it’s useful: Built for modern web development, especially JavaScript frameworks like React, Vue, and Angular, with live collaboration features.

6. JS.do

https://www.js.do/ Languages: HTML, CSS, JavaScript Why it’s useful: Provides a simple interface for quickly testing JavaScript code snippets and seeing immediate results.

7. Ideone

https://ideone.com/ Languages: 60+ (C++, Python, PHP, Java, and more) Why it’s useful: A powerful online compiler that supports a wide variety of programming languages. Great for quick testing across multiple environments.

8. Paiza.io

https://paiza.io/ Languages: Multiple (Python, JavaScript, Ruby, C++, Java, etc.) Why it’s useful: Allows you to write and run code in various languages and even create web apps.

9. PlayCode

https://playcode.io/ Languages: HTML, CSS, JavaScript Why it’s useful: An online JavaScript playground that allows live coding with instant previews, perfect for experimenting with front-end code.

10. Dabblet

https://dabblet.com/ Languages: HTML, CSS Why it’s useful: A clean, minimalistic tool for testing HTML and CSS with live previews. It’s perfect for quick testing without any distractions.

11. Glitch

https://glitch.com/ Languages: JavaScript, Node.js, HTML, CSS Why it’s useful: A collaborative online platform for creating, remixing, and sharing full-stack web applications. Great for building prototypes quickly.

12. CodeSandbox

https://codesandbox.io/ Languages: JavaScript, React, Vue, Node.js, TypeScript, and more Why it’s useful: Perfect for building, testing, and sharing web applications. It provides instant feedback and supports popular frameworks.

13. TIO (Try It Online)

https://tio.run/ Languages: 60+ languages (Python, C, Java, PHP, Ruby, and more) Why it’s useful: A lightweight online interpreter for quickly testing code in various languages without setting up a local environment.

14. Google Colab

https://colab.research.google.com/ Languages: Python (Supports Jupyter Notebooks) Why it’s useful: A cloud-based environment for writing and executing Python code, especially useful for data science and machine learning projects.   These websites provide instant environments for writing, testing, and debugging code, making them highly useful for both learning and professional development.