As you're probably aware, web-based retailers like Amazon track not just your purchases but also your browsing habits through their sites. They correlate the browsing data from all of their customers with their purchasing habits and use this to provide recommendations along the lines of "Customers who purchased the Opti-Max 5000 Butt Plug also purchased 55-gallon drums of Anal-Eze lubricant!" You get the idea.
It's an interesting bit of realtime data mining, and I think that it has uses beyond simple marketing. I think that the technique lends itself very well to personality analysis. For example, today I was told by Amazon that customers who bought items in my recent history also bought the following two items:
New Methuselahs, a article about medical technology in the 21st Century by Dr. Gregory Benford
Super Monkey Ball Banana Blitz, by Sega
Yup. If my personality can be summed up by the juxtaposition of two products then I think they've just about nailed it. :-)
Oh, as a little bonus, check out this item on Amazon!
I think that the free shipping really sells it for me. Ooh! And be sure to read the reviews at the bottom. They're a riot! :-D