Another feature is the ability to create "shelves" for the books you read. I have created a "shelf" for my book club books, 2010, fiction, nonfiction, young adult literature, and children's books. And I can classify each book into more than one category.
In addition, you can rate the books you read and write reviews if you are so inclined. Goodreads also allows you to automatically update twitter when you rate a book, again, if you so choose. And it allows you to post your reviews to your blog (like this one).
You can also connect with friends on Goodreads and share reviews, book lists, comments, and more, or you can join book discussions. It's quite a handy website for readers!
(I was not compensated in any way for promoting Goodreads, but I wish I could be!)