I know, this is old news, but I have been giving A9.com another look. Privacy advocates probably do not like A9.com because the first thing the site does is to read your amazon.com cookie so A9.com knows who you are.
A9.com maintains a search history and is especially nice when search results are from inside a book – you can often read relevant pages out of books. A9.com uses Google, the internet movie database, the amazon.com database, and other information sources.
I have been using the Google SOAP web services API for about 18 months – very cool to be able to use Google easily in my own programs. I would like to see Amazon offer something similar. Amazon does have both a SOAP and a REST API for accessing its main site, but I don’t think that this is tied in with their search site yet.