"A Mother's List of Books" by Theresa Fagan - this is a link to where you can purchase this useful 35-page booklet. It gives ratings and recommendations of books divided by age group (opens in a separate browser window).
A Catholic Mom's Guide to Books: in blog form, including matters other than books as well; 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)
http://greatbooksforchildren.weebly.com/: This is a nice website with short, categorized lists of books, to encourage reading at a young age. At the time of posting, the lists are of books thought to be especially attractive to boys, though it promises to post more lists for girls eventually. (Of course, you might quibble with this categorization: many girls like the type of adventure book this site lists for boys.)