LibraryThing is a nifty little tool kind of like Facebook for literature. You can, essentially, upload your entire library "catalog" onto the internet. You find your book using Library of Congress, Amazon, or any other "database"; you clicketyclick; you choose what information you want; you pick a nice cover; you done. It's strangely addictive.
For people like me, who have long kept track of their book collection with a lame-o Word document, this is the bee's knees. The visual stuff is really cool: each book can have its own cover image. Authors can be listed in "clouds" (the more books per author, the bigger the author's name; this shows you at a glance what kind of groupie you are). And--here's where the social networking bit comes in--you can also see related libraries: whose collection is most similar to yours. You can tag your books, you can join groups, you can upload LibraryThing to your cellphone. No more buying multiple copies of the same book! (Which I tend to do on occasion.)
That being said, there are a couple of things I don't like. You can organize your collection alphabetically by author, or you can order it alphabetically by title...but you can't do both: author first, alphabetically by title. This annoys the heck out of me, especially when I have unusually prolific authors--it makes finding things difficult. I don't think you can order your collection by call number either--not that I would, but it would be nice to have the option. (Why have that information available if you can't use it?)
The other thing: okay, really, when are you ever going to use this? Like Facebook, it's a fun procrastination tool--the fact that I spent more than four hours on it is proof of its pervasive appeal. But unless you're the sort of person who compulsively organizes your spice rack--nothing wrong with that, I'm just saying--you might give this a miss.
My LibraryThing "catalog" can be found here--having spent all that time on it, I figure I should at least show it off--conversely, you can see what kind of trashy taste I have in novels. ;P I was doing it all from memory, though, so the ISBNS and publication dates can't be trusted, and it's not a complete listing by any means--but the titles, authors, and cover images are accurate!
ETA: The biggest group on LibraryThing? Librarians who LibraryThing. ::facepalm:: Go figure.