We recently found out about this awesome online book adventure club. You can find it here.
I believe the site is promoted by Sylvan Learning and it's purpose is to encourage young readers. My kids are so competitive (I don't know where they get that from...) and they love the computer, so this type of thing works really well for us. The site gives book lists and quizzes on the books. We have found that a lot of the books we have in our own library have quizzes available. The kids earn points for reading and being quizzed on the book, and eventually will get prizes.