Kathy Schrock has an extensive guide to the best websites. This particular page summarizes the sites she has recommended in the past. (It started more as daily, and is more like weekly now.) Her entire site is very organized, and has something for every discipline.
This blog, started by two New York moms, reviews many children's books. They are searchable by title, author, reading level, and more. Their rating system is a bit different than many other (such as levels of humor)