1. Open Culture: Not a large a selection, but high quality texts. If you just want to skim a book to brush up on a course you took in ninth grade, download one of these. I have yet to be disappointed.
2. Book Boon: Provides free college-level textbooks in a PDF format. Probably the widest range of subjects on the web. The site is also pretty.
3. Flat World Knowledge: The worlds largest publisher of free and open college textbooks. Humanitie texts are particularly difficult to come by, this site has a great selection in all disciplines.
4. Textbook Revolution: Some of the books are PDF files, others are viewable online as e-books, or some are simply web sites containing course or multimedia content.
5. Library Pirate: I’ve always had an addiction to torrent based pirating. When this site opened a few months ago, I went a little overboard. After dropping two hundred on a paperback spanish textbook, I downloaded the ebook version illegally. I also got a great Psyc text i’m obsessed with. It will be interesting to see how this site grows- they already have a great selection.