Some websites that I like and recommend to check it out:
Fish Eaters: Traditional Catholic Resources
The Project Gutenberg: Online Library with Free Ebooks
Sci-Hub: Download scientific papers without paywall
Library Genesis: Download Books for free
Aozora Bunko: Online Japanese Library: Download or Read Japanese Books copyright free
Debian: The Universal Operating System
Arch Linux: The best Linux Wiki on the internet
Suckless Software: Site of many interesting non-bloated software
Landchad.net: Learn to Self-Host your own services
Teach Yourself CS: Books recommended to study fundamentals of Computer Science
Dig Deeper: Website
Project Euler: Many interesting challenges that will help you to learn math and programming
Set Theory: Website to learn Math