A fairly new site called RateMyBookCover offers the ability to do just that -- you upload a cover file and let the community vote on whether they like it or not. Are you an author who doesn't like the cover design of your upcoming book? Post it on RateMyBookCover and see if the community agrees with you.
The site currently has a small number of covers available for rating but it's sure to grow in the coming months. I'd also like to see them add a comments feature so everyone can see why a book got a good/bad rating, not just the overall rating itself. Finally, it would be nice if they'd offer a way to rate multiple designs of the same cover as well as related covers.