I’ve been using [LibraryThing] to catalog my books since September 2005. All Things Considered on NPR recently featured LibraryThing in a story called “Web Sites Let Bibliophiles Share Books Virtually” (listen).
I especially love the intro to this story:
You know that you’re a bibliophile if you check out peoples’ bookshelves when you visit their homes, if you never pass a used bookstore without going in, or if you have a giant wishlist on Amazon.com.
That fits me to a tee. If it fits you, you owe it to yourself to check out LibraryThing! 
(hat tip: The LibraryThing Blog)