"LibraryThing is an online service to help people catalog their books easily. You can access your catalog from anywhere—even on your mobile phone. Because everyone catalogs together, LibraryThing also connects people with the same books, comes up with suggestions for what to read next, and so forth.."
Not sure if this is the mother or the mother-in-law of all book sites but it sure is huge.
[Thanks to Catherine Dolginko]