here are some free useful websites for studying computer science

Learn X in Y Minutes - Offers short, concise tutorials for various programming languages and tools.
CS50 - An introductory computer science course from Harvard University.
CS50G - An Introduction to Game Development by Harvard.
CS50W - Introduction to Web Development with JS and Python by Harvard.
CS50AI - Introduction to AI with Python by Harvard.
CS50P - Introduction to Python development by Harvard.
CS50 Cyber Security - Cyber Security course by Harvard.
Developer Roadmap on GitHub - A comprehensive guide for developers at all levels.
Cisco Skill for all - Offers free courses from Cisco.
suckless - A community-driven project developing minimalist software tools and utilities.
Kernel Documentation - Provides documentation for the Linux kernel.
OSDev Wiki - Offers tutorials and resources for operating system development.
Codewars - Challenges to improve coding skills.
LeetCode - Similar to Codewars but with a more algorithmic approach.

here are just some cool websites:

Google Scholar - A freely accessible web search engine that indexes the full text or metadata of scholarly literature.
PubMed - A free search engine accessing primarily the MEDLINE database of references and abstracts on life sciences and biomedical topics.
Sci-Hub - Provides free access to research papers.
ArXiv - An open-access archive for scholarly articles in the fields of physics, mathematics, computer science, quantitative biology, quantitative finance, and statistics.
NASA Technical Reports Server (NTRS) - Provides access to NASA’s current and historical technical literature.
512 kb - A collection of web pages that are designed to be 512 kilobytes or less in size.
diagrams.net - A free online diagram editor.

Hack The Box - An online platform offering virtual labs and challenges for practicing hacking skills.
CTF101 - Provides resources and tutorials for learning Capture The Flag (CTF) competitions.
YesWeHack - A bug bounty platform to report security vulnerabilities.
Bugcrowd - A crowdsourced cybersecurity platform.
HackerOne - A bug bounty platform for reporting security vulnerabilities.
VulnHub - Provides materials for practicing penetration testing.
OpSec101 - Provides resources and tutorials for operational security.

Marp - Markdown presentation ecosystem.
LaTeX - A document preparation system often used for technical or scientific documents.
HTTrack - A free and open-source web crawler and offline browser.