"This is a listing of 27 sites that legally offer free textbooks and written course material.
There is a separate page with links to free audio and video courses.
To the best of my knowledge all of the websites listed here offer only content that they are legally entitled to transfer to you. If you find a site listed here that you know illegally offers copyrighted material, please contact me through the site with a link to the infringing content."
Via Beth Dichter