www.goodtoread.org: (opens in a separate browser window) a website with reviews and evaluations of books for kids, well organized and searchable.
A Catholic Mom's Guide to Books: in blog form; not clear how often it is updated, but the reviews are thorough and provide an interesting perspective (opens in a separate browser window).
http://canread.pbworks.com/: Over 1400 titles recommended for grades K-12. Use the "sidebar" - not the "navigator" - to search by grade level, author, or title. (opens in a separate browser window)