Google has rolled out My Library, which looks a lot like a skeletal version of LibraryThing plus an anonymous corporate skin and minus community spirit.
LibraryThing is a massive, participatory catalog of people's private libraries, enabling people to share, tag and review the contents of their collections, collect and contact like-minded people (what better way to find like-minded people than browse their bookshelves?), and put together neat stats.
As of this morning, 265,965 members have catalogued 18,261,561 books. We like LibraryThing and its friendly and open feeling, and hope it lives long and prospers.