LibraryThing.com is a web site that allows you to catalog the books that you own, and even allows you to make a wish list for your friends and relatives can buy you a gift. The web page includes:
Enter what you're reading or your whole library — it's an easy, library-quality catalog. Because everyone else is doing it too, LibraryThing connects you with people who read the same things.
* Searches Amazon, the Library of Congress and 45 other world libraries.
* Show it or keep it private. Put your books on your blog too.
* Get recommendations. Connect to people with all your obscure stuff.
* Tag your books as on Del.icio.us and Flickr (eg., wwii, magical realism, vampires, theology, dogs, philosophy of science).
* Export your data. Import from almost anywhere too.
* Enter 200 books for free, as many as you like for $10 (year) or $25 (life).
I haven't done this yet, but will do it soon. I'll share the URL on my blog when I do it.