Book Search is a program created by Kevin Solway that allows you to search for a book simultaneously at (among others) Amazon, Barnes and Nobles, Powells, ABE Books, and Bookfinder.com (which means you will also be searching Alibris.) You can search by author, title or keyword. What's nice is that Book Search allows you to view all these sites simultaneously by splitting the window into columns or rows. Here is an image of the search interface.