Yahoo! (In the sense of joy, not an internet search engine.) I love Library Thing.
I enjoyed exploring the Zeitgeist - see who the worst- and best-rated authors are, the 25 most-rated books and more. And check out the largest libraries.
On the search page you can do searches on usernames (hint: have fun doing a search under "librarian" and see all the users who come up and then look at their lists).
I also had a lot of fun adding books I've read - the only drawback is that you can only have a hundred books on the free account. Total bummer as I'm sure most library staff will fill personal accounts up quickly!