* OpenStax: OpenStax is a non-profit organization that provides free textbooks and other learning materials. They offer a variety of notes and study guides for different subjects, including math, science, history, and more.
* Coursera: Coursera is an online learning platform that offers courses from top universities around the world. Many of these courses come with free notes and study guides that you can access even if you don't enroll in the course.
* edX: edX is another online learning platform that offers courses from top universities around the world. Many of these courses come with free notes and study guides that you can access even if you don't enroll in the course.
* Khan Academy: Khan Academy is a non-profit organization that provides free online education. They offer a variety of notes and video tutorials for different subjects, including math, science, history, and more.
* Wikipedia: Wikipedia is a free online encyclopedia that contains information on a wide range of topics. You can find notes and study guides on many different subjects by searching Wikipedia.
* Reddit: Reddit is a social media platform where users can share and discuss a variety of topics. You can find notes and study guides on many different subjects by searching Reddit.
* Quora: Quora is a question-and-answer platform where users can ask questions and get answers from other users. You can find notes and study guides on many different subjects by searching Quora.
* Quizlet: Quizlet is a website and app that allows users to create and share flashcards. You can find notes and study guides on many different subjects by searching Quizlet.
* Google Drive: Google Drive is a cloud storage service that allows users to store and share files. You can find notes and study guides on many different subjects by searching Google Drive.
* GitHub: GitHub is a website and app that allows users to share code and other projects. You can find notes and study guides on many different subjects by searching GitHub.