BookDB

With BookDB you can enter all your books with author, category, publisher etc and print them out in a variety of formats.
If you're always going to book sales and buying bargains only to get home and find you've doubled up, take a printout with you! BookDB has a super-condensed print mode so you'll need a magnifying glass, but at least it'll save a tree or two.

BookDB also has lending library features. With them, you can add borrowers, loan out books, add multiple copies and so on.