I've mentioned the covers of the Newbery winners more than a few times in my reviews here. Usually when the new covers are worse than the originals, or the covers are misleading (they made me think the book was going to be awful, and it was great, or vice versa).
Here's a blogger who is designing new covers for all the winners, starting with The Story of Mankind in 1922. He's up to 1928 now (Gay Neck), and it's pretty interesting looking at the book with a modern YA-ish style.
I can't wait until he gets to some of the more recent and/or classic ones! There are a few that I think cannot be improved upon. What do you think?