Lean
- Real Analysis, The Game - Great Lean project for learning the basics of real analysis
- Natural Number Game - The best introduction for Lean, giving a good overview of basic syntax and some of the language works
- Lean Community - Lean proof assistant resources
- Lean Game Server - Some popular Lean games
Russian
- Amazing Russian - Channel with great videos for beginners and intermediate speakers, better explanation of grammar than most other channels
- Russian Lessons . net
- Russian for free
French
- Le Point du FLE - Massive collection of exercises and lessons from many different websites and channels.
- French with Alexa
Programming
- GitHub - Code hosting and version control
- Stack Overflow - Programming Q&A
- MDN Web Docs - Web development documentation
Learning Resources
- Khan Academy - Free online courses
- Coursera - Online learning platform
- MIT OpenCourseWare - Free MIT course materials
Mathematics
- Wolfram MathWorld - Mathematics encyclopedia
- arXiv - Research paper archive
- Math Stack Exchange - Mathematics Q&A
- 3Blue1Brown - Math visualization videos
Other Resources
- Project Gutenberg - Free ebooks
- Internet Archive - Digital library
- Google Scholar - Academic search engine