I recently became addicted to the never-ending book quiz on Goodreads. Unfortunately, it lies and it just ended for me, but since members of the site are adding new questions every day, I'm sure there will be more before too long. Not all the questions are science fiction and fantasy related (in fact, very few are) but it was kind of fun to see what I remembered from high school literature and books I read a long time ago. I was amazed at how much useless trivia I remember from reading "The Baby-sitters Club" books as a kid.
If you haven't checked out Goodreads, it's a great place to read book reviews, meet other book-a-holics, keep track of your books, and simply check out what your friends are reading. I also use it to view what books I have available to choose from that I have not yet read when trying to decide what to read next. It's a lot of fun, as is LibraryThing.
LibraryThing is also a great place for keeping track of your books, and it tends to have more active forums than Goodreads for discussing various book-related topics, such as fantasy, science fiction, historical fiction, nonfiction, and so on. You can also get recommendations based on books you own or even anti-recommendations (which are semi-accurate, kind of like Amazon recommendations). It's also a good place for reading book reviews. LibraryThing is less social than Goodreads, although I don't actually have friends on LibraryThing so I'm not sure how that part of it compares.
One of my favorite recent LibraryThing discoveries is the Early Reviewer books. Every month the option is available to request books to review, and if you are lucky, you may get one of these books. Early Reviewer books are either relatively new releases or books that have not yet been released. My copy of Last Dragon was from this program, which was quite a surprise since there were only 15 copies and 800 people requesting it. Unfortunately, I didn't get a copy of Neal Stephenson's Anathem, which was one of the books they were giving away this month.
Of course, Amazon is also a fantastic way to waste time with all the books to drool over and add to the wish list, reviews to peruse, and author blogs to read. Neil Gaiman, Robin McKinley, Elizabeth Bear, Tobias Buckell, Brandon Sanderson, Carol Berg, and Daniel Abraham all have blogs on Amazon Daily.
Does anybody know any other fun book sites?