I suspect that almost everyone knows about this one, but just in case:
Project Gutenberg is a not-for-profit online library of English books which had lost the copyright due to time elapsed from the publishing date. If you want to read Jane Austin, Mark Twain, Stevenson or Swift in original - they're all here. There are also some books in other labguages, but the collections are, predictably, much smaller.
www.gutenberg.org/wiki/Main_Page