I recently found out about LibraryThing, a website that lets you catalog your books online, and started entering my books. It's really easy to enter a book even with partial information. Just entering a partial title or the name of the author brings up a list of possible matches, and simply clicking on a book from that list will add it to your library with all the available information: ISBN, title, author, publisher, year of publication and so on. You can also tag your books so they can be searched by keyword, rate them, and write comments. And a social networking system shows you other people who have the same books and suggest books you don't have and might be interested in. Pretty cool.