I'm borrowing this from Five Cent Nickel on PFBlog.com, where I just read about it. I tried it today, and it actually worked, as advertised!

Say you're out at a bookstore and wonder if you can get a book cheaper on Amazon. Or you're at a library booksale or yard sale and wonder if a book is worth trying to resell. What you do is call this number on your cell phone: 617-712-3574 and when prompted, enter the ISBN number from the book. It tells you the list price, Amazon's price for new, and the lowest used price listed by Marketplace sellers. Today at the booksale, it saved me from spending money on quite a few books that are only selling for pennies on Amazon. I was able to keep my spending to only $3, which included two books I wanted for myself, plus several I can sell for a total of about $18. And I don't have to go lugging a bunch of unsellable books to Goodwill, after lugging them all home in the first place.

If you want to buy a book you looked up through their number, they ask that you go to their website and order it through their interface so they get a small affiliate fee back from Amazon. In any case, it doesn't cost YOU anything but the cell phone call. I'm very happy with it--especially since I just saw an ad for a similar business that wants $4.99 a month for the service.