80: Hathi Trust
73: Internet Archive
67: Project Gutenberg
66: Google Books
50: Sacred Texts Archive
50: FREE Kindle ebooks
40: LibriVox audio
In addition to these free sources, there are appx. 40 books with cheap Kindle ebook versions ($0.99-$2.99).
Before school starts, I hope I can write up a kind of "tips" page on which online options are best suited to which purposes (search, reading, downloading, etc.).
I am so glad and proud that my school is a member of the Hathi Trust!!! The site is incredibly useful for all users, and even more useful for folks who are members of the consortium (extra download privileges is the benefit I most enjoy).