LibreTexts: (University of California, Davis) Mostly undergraduate university textbooks. “The LibreTexts mission is to unite students, faculty and scholars in a cooperative effort to develop an easy-to-use online platform for the construction, customization, and dissemination of open educational resources (OER) to reduce the burdens of unreasonable textbook costs to our students and society.”
Open Culture: Links to free online textbooks, courses, etc.
Open Educational Resources Commons: K-12 and university.
OpenStax (Rice University): Mostly first-year university and AP high-school resources.
Open Textbook Library: “Open textbooks are licensed by authors and publishers to be freely used and adapted. Download, edit and distribute them at no cost.”
Project Gutenberg: “Project Gutenberg is an online library of free eBooks. Project Gutenberg was the first provider of free electronic books, or eBooks. Michael Hart, founder of Project Gutenberg, invented eBooks in 1971 and his memory continues to inspire the creation of eBooks and related content today.”
Internet Archive: “Internet Archive is a non-profit library of millions of free books, movies, software, music, websites, and more.”
Mathematics
American Institute of Mathematics Open Textbook Initiative: “The American Institute of Mathematics (AIM) seeks to encourage the adoption of open source and open access mathematics textbooks. The AIM Editorial Board has developed evaluation criteria to identify the books that are suitable for use in traditional university courses. The Editorial Board maintains a list of Approved Textbooks which have been judged to meet these criteria.”
Discovering the Art of Mathematics: Mathematical Inquiry in the Liberal Arts: “The DAoM library includes 11 inquiry-based books freely available for classroom use. These texts can be used as semester-long content for themed courses (e.g. geometry, music and dance, the infinite, games and puzzles), or individual chapters can be used as modules to experiment with inquiry-based learning and to help supplement typical topics with classroom tested, inquiry based approaches (e.g. rules for exponents, large numbers, proof).” They also list links to other inquiry-based learning materials here.
An annotated list of free mathematics textbooks from math.stackexchange with lots of additional links.
List of online mathematics textbooks (George Cain, Georgia Tech).
List of calculus and pre-calculus textbooks (Sue VanHattum).
Physics
An annotated list of free physics textbooks from physics.stackexchange, with some additional links.
The Feynman Lectures on Physics: A classic undergraduate textbook from the 1960s, typeset in LaTeX. Challenging and insightful.
Light and Matter: An introductory physics textbook by Benjamin Crowell.
Motion Mountain: A quirky but very interesting physics textbook written by Christoph Schiller. “Wonder and thrill on every page.”
The Physics Hypertextbook: Written and illustrated by Glenn Elert.