Open Library

Open Library's stated goal is to have "One web page for every book ever published." Not too grandiose, is it? Then again, who'd have thought that Wikipedia would take off the way it did?

The project is in need of contributors who can provide book information, among other things. It promises to be a gold mine of information for collectors of all sorts of books, so head over there and see if you can add to the general store of knowledge.