Librarything! How I love librarything! I joined weeks after the site went live...even gave a dataset to Tim to play with while developing the thing. I took a marc dump from our current library catalog (about 10, 000 books) and exported it all to librarything. I use the tagging feature, use the feeds for a "what's new", and have encouraged LT members to become Quatrefoil members, and Quatrefoil members to go on LT.
The one drawback is we have a large DVD collection, so LT leaves out about half of our library. And we have magazines, and music, and games, and older "historical erotica" which are not represented in our LT catalog. Take a look: