I just learned about LibraryThing from reading Sacha Chua's blog post, "Taking book notes." LibraryThing is a social network for book lovers.
The general idea is that you enter books into your library by searching for a title or ISBN. There's also an import tool that will scan a webpage and pull in the ISBNs it can recognize. I used this to import the 106 books that were referenced on my "List of Books I've Read Recently," which I might decide to replace with some references to my library on LibraryThing.
Once you have books in your library, LibraryThing will provide links to profiles of users that have many of the same books in their libraries. You can navigate to their libraries and leave comments on their profiles.
Well, that's just a brief overview. Check out LibraryThing for yourself.
For a more in-depth discussion of LibraryThing and other such sites, check out NPR's All Things Considered discussion of social networks for book lovers.