Sometimes referred to a place where one can find one's textbook for free. I wouldn't count on that; surveying the Mathematics section, I found it limited.
Still, considering what one pays for Math and Science books, a page of free downloads may be worth one's while.
Scanned copies of manuscripts of historic interest, such as a first edition of Shakespeare's sonnets, a few illuminated manuscripts from the Middle Ages and copies of Poor Richard's Almanac
Free e-books at the University of Michigan, with one annoying feature - almost everything is reached by search, not by menu, which might be how some librarians approach libraries, but not really how anybody else does. Such a design eliminates the digital analog of the experience of walking into the stacks and just running into a book.
Still, it is free reading and that is always of interest.