EuropeanaEurope's answer to Google Books. Includes videos and images too.
Google BooksIndexes the full text of a growing number of books. For some books complete text is online; for others only a small portion. No printing or downloading.
HathiTrustFull text books & journals digitized by HathiTrust partner libraries, Google, the Internet Archive, etc. Includes searchable access to over 10 million volumes total; 3 million public domain volumes are fully viewable online and allow downloading one page at a time. Complements Google Books.
WorldCat (all dates)Extensive catalog (OCLC) of books, serials, media, web resources, and more worldwide.