Let me tell you how much I love Goodreads.
Goodreads is pretty much a gigantic database of books. Once you set up your (free) account, you can start putting virtual books on your virtual shelves: the defaults are read, currently reading, or to read. Every time you mark a book as "read", you have an opportunity to rate that book from 0-5 stars, and you'll want to take this opportunity, because once you've provided some ratings, Goodreads will start giving you recommendations based on what you've enjoyed in the past. You can also write a review which will help other Goodreads members decide whether they would like the book.
In addition to the above-mentioned shelves, you can also create your own shelves based on subject matter or other criteria. For example, you may want a shelf for all your historical fiction, or a shelf for all your memoirs. Because this is a virtual shelf rather than a physical one, each book can be on more than one shelf. For example, you may have a book on the "read" shelf that also exists on the "history", "non-fiction", and "World War II" shelves. In addition to being really appealing to those of us with OCD, this functionality allows you to ask Goodreads to show you all the books on any given shelf - e.g., all books marked with "history". I also have shelves for things like "read for school" so that no one thinks I actually read "Portrait of the Artist as a Young Man" on purpose.
I love being able to have an enormous, ever-growing, easily updated and always accessible list of books I want to read. (My to-read list is currently at 457 books.) I am a voracious reader and am always looking for a great book to sink my teeth into. Goodreads offers a lot of excellent ways to grow your to-read list: in addition to the personalized recommendations mentioned above, they send out monthly newsletters featuring author interviews and suggestions, and you can also sign up to be notified when authors you've rated highly release new books. Thanks to this feature and the online request system at my local library, I've been able to read brand-spanking newly-released books right around the time they hit the bookstore and the general public finds out about their existence (if, that is, they happen to walk by a bookstore).
There is also a social media aspect to Goodreads, in that you can "follow" other members and get updates as to what they are reading. If it turns out you have similar tastes in books, you might get some good recommendations this way. You can also join "book clubs" to read the same book as a far-flung group of people and participate in online discussion about it. Goodreads also holds Q&A sessions with authors where members can submit questions for the authors to answer. I haven't explored these aspects very much but I'm sure lots of people really appreciate these functions.
Goodreads is easy to use and has become an incredibly valuable and fun tool for me. If you like to read, chances are you will find something at Goodreads that will make your reading experience even more enjoyable. Come visit!