"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."
Their current library of subjects. I skimmed some of them, and it's a work in progress, but impressive nevertheless.
“It is not knowledge, but the act of learning, not possession but the act of getting there, which grants the greatest enjoyment.” —Carl Friedrich Gauss
"If teaching is reduced to mere data transmission, if there is no sharing of excitement and wonder, if teachers themselves are passive recipients of information and not creators of new ideas, what hope is there for their students?" —Paul Lockhart, A Mathematician's Lament: www.maa.org/external_archive/devlin/LockhartsLament.pdf