Books is a free tool that can help you organize your book collection, acting as a sort of iTunes for physical books. This "virtual card catalog" lets you classify all the books you own using predefined fields (like title, author, publication date, etc.) or you can create your own. Books also lets you track and store summaries, cover images, personal reviews and ratings, notes on multiple copies of a single book, and more--and it even helps you track a book's lending history. "Smart Lists" (similar to iTunes' Smart Playlists) let you group books by any number of criteria (for example, by author, genre, or publication period).
By far the most time-saving feature in Books, though, is its ability to "quickfill" fields, using databases like Amazon's. That way, you can type in a book's ISBN, and Books will autofill much of the book's metadata and even pull in a cover image. Books also gives you export options in case you want to use your book list elsewhere, letting you export into HTML, PDF, onto your iPod, or even just as tab-delimited text, in case you want to drop it all into Excel or export to some other app.
In addition to being free, Books is also open source; if you're a developer, the sky is the limit for tweaking Books and creating localizations and custom plug-ins. The conscientious and responsive developer behind Books has moved onto other projects for now, but even in its current state, Books remains a solid option and a good value for users looking for this kind of app. Anyone experiencing stability issues should download the last stable release (3.2.4 for OS X 10.4+; 2.3.5 for OS X 10.3.9).