LibraryThing is a very interesting site. I love looking at my books online and reading and writing reviews about books I have found to be interesting. I love it how the site recommends more books you might like. It is similar to LastFm which shows your music library through "scrobbling" the music that you listen to.
I also think it is great how it tells you libraries that are similar to yours. It's fun :)
I think that this sort of technology is great and would be very useful to libraries as it is very interactive and fun. I can't wait to get more time to get my whole library up