The National Geographic Kids Blog called Dogeared is great for Book Reviews. Your class can read reviews created by the National Geographic Bloggers, who are a range of different ages, and share their opinion about the book. You could challenge a group of children in your class to write their own review of one of the featured books.
Search the site for specific books, register to comment.
There are also links to blogs with different purposes which can be accessed from the main blog page: http://kidsblogs.nationalgeographic.com/blogs/
This website Book Adventure is a free reading motivation site for Primary aged children which involves quizzes on different books the children have read: http://www.bookadventure.org/
You need to register to access the quizzes but it is free.
The next site is http://www.readwritethink.org/ and is a terrific site with lesson plans and student interactives to support learning in the classroom. I used the Compare and Contrast planner to make a plan to help me write a comparison. I took screenshots of the first and last steps involved to illustrate the process: