Books and Booksellers
AbeBooks (International network of second-hand booksellers)
Biblio (Another international network of second-hand booksellers)
Bookfinder (Search for second-hand and out-of-print titles)
Biblioz (Also searchs for second-hand and out-of-print titles)
Open Library (Wikipedia style catalog of books, some downloadable)
Internet Archive (Digital library offering free access to books, movies, music and web pages)
Google Books (Online catalog of books, including current titles; some can be previewed/downloaded)
HathiTrust (Large-scale repository of digital content from research libraries; some can be previewed/downloaded)