Anybody who has ever spent too many hours on Amazon or Netflix rating and browsing will love LibraryThing, a service where you can choose all the books you own from lists, rate them, tag them, review them (all optional) and then share them with other users or get recommendations based on what other users who share similar collections have in their libraries. Call it social networking for bibliophiles.
As others have wondered in discussing del.icio.us, Technorati, and Flickr, who would have thought that it would be cool to catalog and index your stuff? Sounds a little geeky to me, but it's actually fun (says something about me, I suppose), and easy since you just select your edition from existing metadata rather than having to type everything in.
Individuals can enter 200 books for free, and then you have to pay $10 a year or $25 lifetime for collections beyond that. The metadata comes from Amazon and from Z39.50 compliant library catalogs. You can even choose which edition of a book you have.
There are apps to import Amazon wish lists, and to export your data to Excel, and to do other cool things. You can put a window to your LibraryThing account on your blog to show what you're reading now, or random titles from your collection. (I'm not sure I would do that, as not everything I read is something to brag about...murder mysteries anyone?)