The latest issue of Sky & Telescope has a short article by astronomy professor Andrew Franknoi on a project to address the high cost of college textbooks. OpenStax, a non-profit based at Rice University, has commissioned a series of new introductory-level texts that are free for anyone to download and can be purchased on paper at cost. There have 20 books so far with more on the way.
The subjects covered include math (pre-algebra through calculus), basic statistics, science (astronomy, chemistry, physics, biology, and human anatomy and physiology), social sciences (economics, sociology, psychology, American government), and U.S. history. The introductory astronomy text was co-authored by Franknoi himself with David Morrison and Sidney Wolff and released late last year.
But there are a good many more free high-quality textbooks out there, some not as up-to-date as those from OpenStax but many that are and that in many cases are valuable even if published a few decades back (such as Richard Feynman's famous lectures on physics, volume 1, volume 2, and volume 3).
There are also free interactive language courses, video lectures, and so on. A general reference can be found at OpenCulture.com.
(One passing gripe: To my annoyance, the list of language courses at Open Culture omits mention of Esperanto, for which a number of good free resources can be found on line, including Duolingo, Lernu.net, and (mainly for advanced students and language teachers)Edukado.net. My guess is that this is based on the popular but silly misconception that Esperanto is somehow not a "real" language.)