The LibraryThing is totally cool. It's free for catalogues of up to 200 books but if, like me, you have well over 5000 books in your house, you will need to pay a one-time fee of $10. Well worth it, I'd say, for all the information it will put at your fingertips quickly and easily! Entering books is easy: all you need is the ISBN, and it fills in everything else for you (except for tags, which are user-defined).
It's not just for the anally-retentive, either: the information could be quite valuable in case of theft, fire, or other damage to your collection.