PxT
My DVD, dog food & PEZ purchase history is probably a wealth of useful information
Long article from the NY Times on Target and how they learn & predict your shopping habits using statistical analysis.  Apparently they can tell when a woman is pregnant and, amazingly, very accurately predict her due date based only on shopping patterns.

This part is the best though; Target knows more about your horny teenage daughter than you do:
About a year after Pole created his pregnancy-prediction model, a man walked into a Target outside Minneapolis and demanded to see the manager. He was clutching coupons that had been sent to his daughter, and he was angry, according to an employee who participated in the conversation.

“My daughter got this in the mail!” he said. “She’s still in high school, and you’re sending her coupons for baby clothes and cribs? Are you trying to encourage her to get pregnant?”

The manager didn’t have any idea what the man was talking about. He looked at the mailer. Sure enough, it was addressed to the man’s daughter and contained advertisements for maternity clothing, nursery furniture and pictures of smiling infants. The manager apologized and then called a few days later to apologize again.

On the phone, though, the father was somewhat abashed. “I had a talk with my daughter,” he said. “It turns out there’s been some activities in my house I haven’t been completely aware of. She’s due in August. I owe you an apology.”

 /via jwz