General Coding
- Codecademy - Interactive coding lessons in various languages.
- Coursera - Free courses from universities (audit mode).
- edX - Free courses from top universities and institutions (audit mode).
Web Development
HTML & CSS
- MDN Web Docs - HTML - Comprehensive resource for HTML.
- MDN Web Docs - CSS - Comprehensive resource for CSS.
- FreeCodeCamp - Responsive Web Design - Learn HTML and CSS by building projects.
JavaScript
- Eloquent JavaScript - A free book to learn JavaScript.
- JavaScript.info - Modern JavaScript tutorial.
- FreeCodeCamp - JavaScript Algorithms and Data Structures - Interactive JavaScript curriculum.
Front-End Frameworks
- React Official Documentation - Comprehensive guide to learning React.
- Vue.js Official Documentation - Comprehensive guide to learning Vue.js.
- FreeCodeCamp - Front End Libraries - Learn popular front-end libraries like React and Bootstrap.
Data Science and Machine Learning
- Kaggle Learn - Micro-courses on data science and machine learning.
- Google's Machine Learning Crash Course - Introductory course on machine learning.
- Coursera - Machine Learning by Stanford - Free access in audit mode.
Mobile Development
Android
- Android Developers - Free courses and tutorials from Google.
- Udacity - Developing Android Apps - Free course offered by Google.
iOS
- Stanford's iOS Development Course - Free iOS development course using Swift, available on iTunes U.
- Ray Wenderlich - iOS Tutorials - Free tutorials on iOS development.
Version Control
- Pro Git Book - Comprehensive book on Git.
- Codecademy - Learn Git - Interactive lessons on Git.
Algorithms and Data Structures
- GeeksforGeeks - Extensive tutorials on algorithms and data structures.
- VisuAlgo - Visualize data structures and algorithms.
- MIT OpenCourseWare - Introduction to Algorithms - Free course materials from MIT.
Coding Practice and Challenges
- HackerRank - Practice coding problems and challenges.
- LeetCode - Coding problems and challenges.
- CodeWars - Practice coding with challenges and kata.
- Project Euler - Mathematical and computational problems.