Occasionally someone will come up with a web app that hasn't got any real benefit in life, the universe and everything, but that's great fun. And surely this is not a bad thing.
Thanks to @thecreativepenn for pointing out Yasiv.com. Put in a product listed on Amazon.com and you will see a diagram of related purchases.
The developer sees this as being a good way to think 'if other people bought this as well, then perhaps I should look at these too.'
But also if you happen to have produced a product that's sold on Amazon, then it is rather fun to take a look at these 'related' products and what other things people bought. It doesn't just apply to books - you can do it with anything on Amazon.com (though not at the moment .co.uk - I emailed the developer to ask, and it's on his to-do list). Give it a try...
Quick update as of 8 February - I'm pleased to say you can now select the Amazon of your choice from the list at the bottom of the screen!