I was actually told about this site a month ago, but haven't had a chance to really look at it. (Although since I did end up ordering about 6 books this holiday, I should have looked at more closely in December!)
The site is BooksPrice.com.
The set-up is you search for what you are looking for and it will let you know the different prices around the internet for you! For fun today I typed in The DaVinci Code and got LOTS of hits back ... the prices, ranged from 3.99 to 32.65. The best part I think - the price includes shipping! So those 2 3.99 prices, are 3.99 WITH shipping. Okay, I really like how it just adds the shipping on the main search screen because then you know the total cost of the book.
It brought back a lot of different places for TDC - used to new.
It also allows you to search for CD's and DVD's as well. I can see how useful this site would be, especially if you are hunting for the a good bargain (which, I most certainly will be needing to do for some time here!).
Anyway, try them out if you need to find something. And if you do use it, let me know how it goes.